Thanks to nice support even for ex-users of AT&T service revoked 4 years ago, iPhone 3GS + iOS6.1 has been SIM-unlocked successfully. This is the link for online application form to request SIM-unlock.
A half day later, AT&T replied back via email to let me know unlock seems to be done. I followed the instruction http://support.apple.com/kb/HT5014 as it was mentioned in their email reply but it didn’t work, restoring iPhone never resolved. Now then I had purchased a prepaid SIM card ( CallYa by vodafone.de) for SmartPhone in Germany where I live now, so that I tried to insert the prepaid SIM card to iPhone and reconnect to iTunes via USB docking cable, then restored again. The key point is to insert a new SIM card different from the original AT&T SIM I had used. iTunes finally shows a blank screen after reconnected, and iPhone screen shows the message “Required to activate” or the like (forgot details…), then I followed the screen instruction as if the iPhone is firstly activated. After it’s activated with a new SIM, carrier settings are refreshed to the new.
Words of today–
remain/stay in limbo