For those of you having activation problems, try these things:
Popping in a sim card from and activated AT&T phone.
Popping your sim card out and back in.
Calling AT&T and saying WTF, they may ask you for your IMEI and sim pin, then bam.
Or you can wait up to two days according to Apple.

Yeah, I had a problem because I didn't know that the 3GS had it's own SIM already in it. Anyway, I called AT&T and the whole thing was straightened out in like 10 minutes.