I have a 3g that has been upgraded to 3.1.2 firmware and jailbroken/unlocked with Blackrain and Blacksnow. I've been getting a very low wifi signal and it takes several minutes to connect.
I know that this is a known issue.
The iphone is still under warranty, but I don't have an AT&T account. Will the Apple store still replace it since it's under warranty? Do they ask you in the store for your phone number or anything? I actually have an old prepaid AT&T sim I could put in it, but it's not active.