I wrote a script to check a bunch of popular domains (or any domain) and see if they have a reverse DNS entry.
http://www.perturb.org/code/rdns_check.pl
The summary is that every one (100%) of the domains I checked had reverse DNS.
digg.com has 7 MX records
** (Good) aspmx.l.google.com = 74.125.65.27 / gx-in-f27.1e100.net
** (Good) alt1.aspmx.l.google.com = 74.125.113.27 / vw-in-f27.1e100.net
** (Good) alt2.aspmx.l.google.com = 209.85.143.27 / dy-in-f27.1e100.net
** (Good) aspmx2.googlemail.com = 74.125.43.27 / bw-in-f27.1e100.net
** (Good) aspmx3.googlemail.com = 74.125.127.27 / pz-in-f27.1e100.net
** (Good) mail.digg.com = 74.125.127.121 / pz-in-f121.1e100.net
** (Good) diggstage01.digg.com = 64.191.203.34 / diggstage01.digg.com
nytimes.com has 4 MX records
** (Good) NYTIMES.COM.S7A1.PSMTP.com = 64.18.6.14 / s7a1.psmtp.com
** (Good) NYTIMES.COM.S7A2.PSMTP.com = 64.18.6.13 / s7a2.psmtp.com
** (Good) NYTIMES.COM.S7B1.PSMTP.com = 64.18.6.11 / s7b1.psmtp.com
** (Good) NYTIMES.COM.S7B2.PSMTP.com = 64.18.6.10 / s7b2.psmtp.com
gmail.com has 5 MX records
** (Good) gmail-smtp-in.l.google.com = 74.125.65.26 / gx-in-f26.1e100.net
** (Good) alt1.gmail-smtp-in.l.google.com = 74.125.113.26 / vw-in-f26.1e100.net
** (Good) alt2.gmail-smtp-in.l.google.com = 209.85.143.26 / dy-in-f26.1e100.net
** (Good) alt3.gmail-smtp-in.l.google.com = 209.85.229.26 / ww-in-f26.1e100.net
** (Good) alt4.gmail-smtp-in.l.google.com = 74.125.79.26 / ey-in-f26.1e100.net
hotmail.com has 4 MX records
** (Good) mx3.hotmail.com = 65.54.188.110 / bay0-mc3-f.bay0.hotmail.com
** (Good) mx4.hotmail.com = 65.55.92.168 / mx4.hotmail.com
** (Good) mx1.hotmail.com = 65.54.188.72 / bay0-mc1-f.bay0.hotmail.com
** (Good) mx2.hotmail.com = 65.55.37.72 / col0-mc1-f.col0.hotmail.com
yahoo.com has 3 MX records
** (Good) mta6.am0.yahoodns.net = 98.137.54.238 / mta-v2.mail.vip.sp2.yahoo.com
** (Good) mta7.am0.yahoodns.net = 74.6.136.244 / mta-v3.mail.vip.sk1.yahoo.com
** (Good) mta5.am0.yahoodns.net = 98.139.175.224 / mta-v1.mail.vip.bf1.yahoo.com
fastmail.fm has 2 MX records
** (Good) in1.smtp.messagingengine.com = 66.111.4.71 / mx2.messagingengine.com
** (Good) in2.smtp.messagingengine.com = 82.145.212.142 / tmx2.messagingengine.com
comcast.net has 2 MX records
** (Good) mx1.comcast.net = 76.96.62.116 / imta.westchester.pa.mail.comcast.net
** (Good) mx2.comcast.net = 76.96.30.116 / imta.emeryville.ca.mail.comcast.net
apple.com has 7 MX records
** (Good) mail-in13.apple.com = 17.254.13.11 / mail-in.apple.com
** (Good) mail-in14.apple.com = 17.254.13.13 / mail-in.apple.com
** (Good) mail-in11.apple.com = 17.254.13.7 / mail-in.apple.com
** (Good) mail-in12.apple.com = 17.254.13.10 / mail-in.apple.com
** (Good) mail-in2.apple.com = 17.254.13.5 / mail-in2.apple.com
** (Good) mail-in6.apple.com = 17.254.13.9 / mail-in6.apple.com
** (Good) mail-in3.apple.com = 17.254.13.8 / mail-in3.apple.com
google.com has 5 MX records
** (Good) aspmx.l.google.com = 74.125.65.27 / gx-in-f27.1e100.net
** (Good) alt1.aspmx.l.google.com = 74.125.113.27 / vw-in-f27.1e100.net
** (Good) alt2.aspmx.l.google.com = 209.85.143.27 / dy-in-f27.1e100.net
** (Good) alt3.aspmx.l.google.com = 209.85.229.26 / ww-in-f26.1e100.net
** (Good) alt4.aspmx.l.google.com = 74.125.79.26 / ey-in-f26.1e100.net
aol.com has 4 MX records
** (Good) mailin-02.mx.aol.com = 64.12.139.193 / mtain-mk.r1000.mx.aol.com
** (Good) mailin-03.mx.aol.com = 64.12.90.33 / mtain-mg.r1000.mx.aol.com
** (Good) mailin-04.mx.aol.com = 64.12.90.34 / mtain-mh.r1000.mx.aol.com
** (Good) mailin-01.mx.aol.com = 205.188.59.194 / mtain-db.r1000.mx.aol.com
wikipedia.org has 2 MX records
** (Good) mchenry.wikimedia.org = 208.80.152.186 / mchenry.wikimedia.org
** (Good) lists.wikimedia.org = 91.198.174.5 / lists.wikimedia.org
ebay.com has 3 MX records
** (Good) lore.ebay.com = 216.113.175.103 / lore.ebay.com
** (Good) data.ebay.com = 66.135.195.180 / data.ebay.com
** (Good) gort.ebay.com = 216.113.167.215 / gort.ebay.com
amazon.com has 5 MX records
** (Good) smtp-fw-33001.amazon.com = 207.171.189.228 / smtp-fw-33001.amazon.com
** (Good) smtp-fw-2101.amazon.com = 72.21.196.25 / smtp-fw-2101.amazon.com
** (Good) smtp-fw-4101.amazon.com = 72.21.198.25 / smtp-fw-4101.amazon.com
** (Good) smtp-fw-9101.amazon.com = 207.171.184.25 / smtp-fw-9101.amazon.com
** (Good) smtp-fw-31001.amazon.com = 207.171.178.25 / smtp-fw-31001.amazon.com
slickdeals.net has 7 MX records
** (Good) ASPMX.L.GOOGLE.COM = 74.125.65.27 / gx-in-f27.1e100.net
** (Good) ALT2.ASPMX.L.GOOGLE.COM = 209.85.143.27 / dy-in-f27.1e100.net
** (Good) ALT1.ASPMX.L.GOOGLE.COM = 74.125.113.27 / vw-in-f27.1e100.net
** (Good) ASPMX2.GOOGLEMAIL.COM = 74.125.43.27 / bw-in-f27.1e100.net
** (Good) ASPMX3.GOOGLEMAIL.COM = 74.125.127.27 / pz-in-f27.1e100.net
** (Good) ASPMX4.GOOGLEMAIL.COM = 209.85.229.27 / ww-in-f27.1e100.net
** (Good) ASPMX5.GOOGLEMAIL.COM = 74.125.157.27 / gy-in-f27.1e100.net
redhat.com has 2 MX records
** (Good) mx1.redhat.com = 209.132.183.28 / mx1.redhat.com
** (Good) mx2.redhat.com = 66.187.233.33 / mx2-old.redhat.com
ubuntu.com has 1 MX records
** (Good) mx.canonical.com = 91.189.94.145 / fiordland.canonical.com
ibm.com has 11 MX records
** (Good) e34.co.us.ibm.com = 32.97.110.152 / e34.co.us.ibm.com
** (Good) e35.co.us.ibm.com = 32.97.110.153 / e35.co.us.ibm.com
** (Good) e1.ny.us.ibm.com = 32.97.182.141 / e1.ny.us.ibm.com
** (Good) e2.ny.us.ibm.com = 32.97.182.142 / e2.ny.us.ibm.com
** (Good) e3.ny.us.ibm.com = 32.97.182.143 / e3.ny.us.ibm.com
** (Good) e4.ny.us.ibm.com = 32.97.182.144 / e4.ny.us.ibm.com
** (Good) e5.ny.us.ibm.com = 32.97.182.145 / e5.ny.us.ibm.com
** (Good) e6.ny.us.ibm.com = 32.97.182.146 / e6.ny.us.ibm.com
** (Good) e31.co.us.ibm.com = 32.97.110.149 / e31.co.us.ibm.com
** (Good) e32.co.us.ibm.com = 32.97.110.150 / e32.co.us.ibm.com
** (Good) e33.co.us.ibm.com = 32.97.110.151 / e33.co.us.ibm.com
calix.com has 4 MX records
** (Good) calix.com.s8a1.psmtp.com = 64.18.7.10 / s8a1.psmtp.com
** (Good) calix.com.s8a2.psmtp.com = 64.18.7.11 / s8a2.psmtp.com
** (Good) calix.com.s8b1.psmtp.com = 64.18.7.13 / s8b1.psmtp.com
** (Good) calix.com.s8b2.psmtp.com = 64.18.7.14 / s8b2.psmtp.com
intel.com has 6 MX records
** (Good) mga02.intel.com = 134.134.136.20 / mga02.intel.com
** (Good) mga03.intel.com = 143.182.124.21 / mga03.intel.com
** (Good) mga09.intel.com = 134.134.136.24 / mga09.intel.com
** (Good) mga11.intel.com = 192.55.52.93 / mga11.intel.com
** (Good) mga14.intel.com = 143.182.124.37 / mga14.intel.com
** (Good) mga01.intel.com = 192.55.52.88 / mga01.intel.com
amd.com has 1 MX records
** (Good) mail.global.frontbridge.com = 216.32.180.22 / mail.global.frontbridge.com
msn.com has 4 MX records
** (Good) mx2.hotmail.com = 65.54.188.110 / bay0-mc3-f.bay0.hotmail.com
** (Good) mx3.hotmail.com = 65.54.188.72 / bay0-mc1-f.bay0.hotmail.com
** (Good) mx4.hotmail.com = 65.55.92.136 / mx1.hotmail.com
** (Good) mx1.hotmail.com = 65.55.92.168 / mx3.hotmail.com
wired.com has 4 MX records
** (Good) samgdem01.advancemags.com = 208.92.44.79 / samgdem01.advancemags.com
** (Good) samgdem02.advancemags.com = 208.92.44.90 / samgdem02.advancemags.com
** (Good) samgnym01.advancemags.com = 69.2.121.12 / samgnym01.advancemags.com
** (Good) samgnym02.advancemags.com = 69.2.121.14 / samgnym02.advancemags.com
cnn.com has 6 MX records
** (Good) hkgmail1.turner.com = 168.161.96.115 / hkgmail1.turner.com
** (Good) lonmail1.turner.com = 157.166.216.142 / lonmail1.turner.com
** (Good) nycmail1.turner.com = 157.166.157.8 / nycmail1.turner.com
** (Good) nycmail2.turner.com = 157.166.157.10 / nycmail2.turner.com
** (Good) atlmail3.turner.com = 157.166.174.56 / atlmail3.turner.com
** (Good) atlmail5.turner.com = 157.166.165.14 / atlmail5.turner.com
msnbc.com has 1 MX records
** (Good) mail.messaging.microsoft.com = 65.55.88.22 / mail.global.frontbridge.com
slashdot.org has 1 MX records
** (Good) mx.sourceforge.net = 216.34.181.68 / mx.sourceforge.net
facebook.com has 1 MX records
** (Good) smtpin.mx.facebook.com = 66.220.155.16 / smtpin.mx.facebook.com
paypal.com has 3 MX records
** (Good) data.ebay.com = 66.135.195.180 / data.ebay.com
** (Good) gort.ebay.com = 216.113.167.215 / gort.ebay.com
** (Good) lore.ebay.com = 216.113.175.103 / lore.ebay.com
amazon.com has 5 MX records
** (Good) smtp-fw-31001.amazon.com = 207.171.178.25 / smtp-fw-31001.amazon.com
** (Good) smtp-fw-33001.amazon.com = 207.171.189.228 / smtp-fw-33001.amazon.com
** (Good) smtp-fw-2101.amazon.com = 72.21.196.25 / smtp-fw-2101.amazon.com
** (Good) smtp-fw-4101.amazon.com = 72.21.198.25 / smtp-fw-4101.amazon.com
** (Good) smtp-fw-9101.amazon.com = 207.171.184.25 / smtp-fw-9101.amazon.com