One look at the Network Coverage board in
AT&T Customer Forums (unfortunately, registration is required) will confirm that AT&T service (and especially 3G) leaves a lot to be desired. I have switched my phone into 2G-mode only a month ago because of how poor the 3G experience has become.
That being said, my PRS-900 does indeed fall back onto 2G (EDGE/GPRS) when the 3G signal is not available. We have quite a few holes in 3G coverage here in Bay Area so it was easy to confirm that the manual is correct in that regard.