I gotta say this is not hardware guys..
I posted awhile ago that my problem was fixed. I had the same issue everyone was complaining about.
I replaced my sim, didn't really help. I had ATT reset my phone or whatever, like send it a new signal so it sees the towers - this did nothing.
One afternoon it just worked again, like a switch was flipped. Keep calling ATT its got to be your area.
I had the same thing where people at work had 3GS and they worked fine where mine loaded nothing. Now my 3G works flawless on my Iphone 4.
Its ATT keep bothering them.
I don't buy this. It's happened to me in 3 separate areas more than 10 miles apart. Other iPhone 4 owners standing next to me did not have the problem. Perhaps it just took a while for the new provisioning on your SIM to finally kick in...?
Damon