AT&T's 2G (called EDGE, 2-300kbps) is as fast as Verizon's 3G. And AT&T has EDGE coverage across it's whole area.
I'm pretty certain that's wrong.
According to Wikipedia, EDGE hits around 240 Kbps.
EVDO Rev A, on the other hand (which is what Verizon and Sprint use) can hit 3.1 Mbps. Which means EDGE is off by about a factor of ten for speed.
ATT is meanwhile in the process of rolling out 7.2 Mbps HSDPA - previously they'd been using 3.6 Mbps, which is pretty darn close to EVDO Rev A for speed.
To make things worse, it seems like real world testing shows wildly varying download speeds on ATT's 3.6 HSDPA, depending on the user load, etc. Which seems to be a common complaint amongst iPhone users.
For references, see http://www.gearlog.com/2009/05/att_rolling_out_hsdpa_72_faste.php or any of the various Gizmodo / Engadget articles about ATT's 3G rollout. You can also check Wikipedia's articles on EDGE and EVDO for bonus points.
(Anecdotally, I've noticed significantly faster browsing on my Spring HTC Hero compared to my HTC ADP1 running on both T-Mobile and ATT's EDGE networks.)
If you must take a computer with you, get a Netbook
I heartily concur with this advice. I spent several weeks traveling in England this summer, and lugging around my 13 inch Macbook is not something I'll repeat. Its too heavy, and too valuable, to really feel comfortable carrying around all day. I'd also recommend getting one of the pre-paid HSDPA modem for internet; Wifi (paid more often than not) does exist, but it ends up being about the same cost to get the cellular modem and the modem is far more convenient. Don't plan on using the computer that much though. I found myself only using it at night to look up the location of where I wanted to go next, and to send the occasional e-mail back home. There's far too many other interesting things to do.
As for transport in and around London, the Oyster card is by far your best bet. The travel / tourist stations in Heathrow can set you up with one, or you can get one at the manned Underground stations. They're cheaper to use than buying tickets on the Underground, although still not cheap (London has one of the most expensive public transit systems in the world). I rarely used the buses; Underground stops are all over the place and I often found that walking was easier (and more fun) when my destination was not directly next to an underground stop. The train system is an excellent way to get around the rest of England (and are another reason to get a cellular modem) and are not terribly expensive. I used them to go on day trips to other cities.
My final bit of advice is to just take some time to wander around. Its surprising the amount of stuff you can stumble across if you're just walking in London. There's a lot of history there, and some really neat sites. I also recommend trying to find some of the open-air markets; they have some great food and are fun to see.
Oh yeah, if you decide to see some of the "traditional" sights in London, be ready for some sticker shock. Westminster Abbey, the Tower of London, and several of the other places tourists go to are very expensive. On the other hand, many of the museums are top-notch and completely free.
The moon is made of green cheese. -- John Heywood