I had this same problem today. ATT and Apple recently unlocked my old original iPhone, as well as my iphone3gs after having them for over 2 years.
The 3GS was jailbroken a year ago and worked fine but now that ATT was offering the unlock I wanted to upgrade the OS.
I went thru the iPhone erase and restore with a third party sim chip, non ATT, and the process worked well with the original iphone just fine.
But last night I erased and restored my " 3GS" with the same non ATT sim chip, ( apples unlock/ restore guide says you can use any working sim chip, so I should not need a working ATT chip right?
( http://support.apple.com/kb/HT5014 ). With the iPhone 3gs, i get the message on the iPhone screen of activation server down. But in itunes i get the message " no sim chip installed" when there is a sim chip.
I am stuck. any help?
NOTE.. the sim chip is clean and the slot looks ok...Prior to unlocking the #GS I had taken the sim in and out, and it was recognized... it was only after restoring with apples new OS for the 3gs that it does not see the sim chip.
I just got it working... heres what you need to do. The problem is that previously jailbroken iphones have lower versions of the Baseband. Apples firmware is now 6.15, and with out that version it won't see the sim card after you unlock and upgrade to iOS 6.x for the iPhone3GS
Follow this tutorial, upgrade to baseband 6.15
Now you will have an iPhone, jailbroken with cydia and 6.15 firmware and the sim will be recognized. Now backup and sync your aps as normal.