I bought an Acer Liquid A1 for $360 on some Google Shopping site (I think there are some for cheaper now. This was about 2 months ago). Not perfect but IMO can't get better for the price (kinda like a cheap Nexus).
It supports the 1900MHz AT&T 3G band, which I use it on, it is currently the stronger AT&T 3G band and has worked fast for me wherever I use 3G (also works on 900MHz/2100MHz 3G bands).
While it is currently on Donut, there are leaked Eclair firmware upgrades (by Acer, kinda beta, works fairly well for everyday use, tho battery life is not much to rave about and couple of minor bugs).
It has a very strong development community on MoDaCo (even has an alpha build of FroYo out tho WiFi/some other things are not working). Already been rooted with several great Eclair-based roms available.
The phone contains:
768MHz underclocked/undervolted Snapdragon (some phones can work at 1GHz, some can't. Can only overclock on Donut as Eclair kernal has not yet been released. Tho I have found 768MHz to be fast enough)
256 Megs of RAM
512 Megs of ROM
3.5" WVGA LCD capacitive touchscreen