"You and I" must be a very selective group. I use my Droid a lot, but in the last 10 months of owning it, I've gone over 500 MB/month once, and hit 400 MB/month 2 months. The rest of the time I haven't gone over 300 MB, and mostly at the 200 MB level. Why?
A good chunk of the time my droid is using wifi (at home). I used to be able to do it at my office as well, but then froyo broke WPA/Enterprise compatibility with cisco access points (well, actually wpa_supplicant broke, but froyo has an older version with this bug). Although android does have many issues with wifi compatibility. Plus, I don't stream music and even with 3G youtube takes a long time to load. And I don't tether my laptop (which is generally prohibited anyway).
On the other hand, I use it as, essentially, a PDA, navigation/gps system, and light browsing (the web browser is good, but I can only deal with so much browsing on a 4" screen. Great for looking something up. Useless for browsing a long time. VoIP may change things, but for the time being I only use VoIP when I'm in locations with wifi but not cell signal.
Personally, I'd be very glad to have a plan like Walmart's. I currently pay $30/month for unlimited data, and use on average, say 400 MB. Over the last 10 months I've paid Verizon $300. With Walmart's plan, I would've paid $160. Even adding on VoIP with a 64 kbit codec (the highest quality SIPDroid offers) for 450 minutes/month adds 216 MB of data usage. (or does it need to be doubled for both ways?)
You're right that the carriers are scared...because it won't be long before real VoIP apps appear for android that anyone can use, and the carriers are reduced to data carriers, not phone providers. Unfortunately, T-Mobile's network is nowhere as good as Verizon's, so I won't even consider it.