Wouldn't it just revert to GPRS if EDGE isn't available? I don't know because everywhere I've been with my iPhone so far has had EDGE support. Granted, even if the iPhone DID work on GPRS when EDGE was unavailable, I don't think I'd have the patience to use it.
And to add to what Celsdogg said, I've had no trouble connecting my iPhone to multiple WIFI networks. It actually acts pretty well as a wardriving utility