I have had the Android Developer Phone (ADP1) for about 9 months now. I have been using it on the AT&T pay as you go plan with pretty good success. So far here is what I have discovered with regard to this combo:
* The phone works fine on the AT&T network for making and receiving calls.
* The AT&T data plan is a pretty good deal. For $20 you get 100MB for 1 month - which lasts well beyond the month for me.
* Downloading and installing both free and paid apps works fine.
* No 3G. I've only had EDGE access. Perhaps I need to re-configure my APN, but I heard that the G1 does not support the 3G frequencies necessary for the AT&T network.
* The annoying "Last Transaction" messages unless you patch it (see my earlier post).
* Android updates need to be manually applied if you want them. You have to go to the HTC website(http://www.htc.com/www/support/android/adp.html), download a couple of files and go through their update process. It's no biggie really, just 1 more thing.
I could go on, but anything else would be universally true for any ADP1 (or presumably a T-Mobile G1).