I just put in a foreign SIM card that I had (bought a Tmo unsubsidized one) and turned on phone. I did all activation on phone...didn't connect to iTunes. It did require a wifi connection. I skipped the AppleID steps and iCloud restore options just set up as new iPhone. It activated and showed roaming on ATT.
Then I shut it off, put in my T-mobile SIM, and connected to iTunes just to do my restore from backup. Everything is working. I haven't put the foreign SIM back in but my understanding is once unlocked always unlocked. I think the need to activate with iTunes and computer went away after the 4S. Also not sure using the different SIM for initial activation is even required....because Apple's database should still indicate it was a device only purchase and the IMEI should have unlocked status even if you didn't go through the process I described. I just did it to be safe and satisfy my OCD.
Then I shut it off, put in my T-mobile SIM, and connected to iTunes just to do my restore from backup. Everything is working. I haven't put the foreign SIM back in but my understanding is once unlocked always unlocked. I think the need to activate with iTunes and computer went away after the 4S. Also not sure using the different SIM for initial activation is even required....because Apple's database should still indicate it was a device only purchase and the IMEI should have unlocked status even if you didn't go through the process I described. I just did it to be safe and satisfy my OCD.