There are 3 possible reasons this can happen:
1. Your antivirus or firewall on your computer is blocking access to Apple's Activation Server at gs.apple.com
2. Your phone is jailbroken or SIM-unlocked by someone other than Apple.
3. Your computer was used to hack or jailbreak some device running iOS, not necessarily the phone with the problem.
For 2. and 3. you can't get help in an Apple hosted forum.
There have been reports of this issue occurring - I for one own a 32GB 3GS and have the same issue (and I have never jailbroken. Try updating to 6.0, and if that doesnt work, restore again. if not, It could be the sim I/O switch in the tray, which a Technician would have to look at for you.