I bought a phone that was advertised as "factory unlocked" so I let Itunes update it because as I understood it it would stay unlocked. After the update I got a message that I had an unsupported sim. so I installed the ATT sim, restored my data and it seemed to go ok. But now when ever I install my old sim (not ATT) I get an invalid sim message. Have I missed a step or was I just sold and unlocked phone that was not actually "factory" unlocked? Is there any way to identify a factor unlocked phone?

Thanks for the help!

It is not a factory unlocked iPhone. I do have one usinf it with a simcard of a non Apple partner carrier. It just activates.

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iPhone 3GS factory unlocked (3.1.2; Blackra1n, Cydia, OpenSSH, custom 3.0 ipcc file (no sig), tethering hack by Dev team, 32 GB)
iPhone 3G (3.0.1; Redsn0w 0.8, Cydia, OpenSSH) 16 GB

After spending hours combing the forums here, I figured I had been had. So unless the Dev guys come up with a fix I guess my option is to go to ATT. Is their any way to tell if the phone is legitimately factory unlocked rather updating it and finding it doesn't work anymore?

The simplest way is to see if ultrasn0w is installed in Cydia. They could have also used purplesn0w to do the unlock.

The definitive way is to restore the phone with a stock OS 3.0 and see if it activates with a non carrier Sim. This can always be reversed because you have not upgraded to a baseband that can't be unlocked.

Your problem was updating to 3.1.x without checking.

Yeah, as they say, hind sight is 20/20. I bought it used and I was a little curious why Cydia was installed on it when I got it, but at that time I didn't know enough about the unlock process to check. And heck, no one on Ebay would ever misrepresent their item, right?