Actually, when a user buys an iPhone, regardless of whatever gripes about the locks that AT&T and Apple have placed on it, the device is the property of the user. You don't even know if he meant to "tinker" with his iPhone other than what was only stated... that the guy placed an older SIM card into the phone and when attempting to undo it, ran into problems.
On the other hand, +when you go and advise him to *play with Legos*...+ ESPECIALLY when the guy is a new poster ... I mean, c'mon... that's just not right, and for iPhonechik to share a laugh with you, fine, but again... the guy is looking for a solution, and neither of you obviously gave him any more than a taunt. Maybe staffo here doesn't know any better that the iPhone is SIM-locked, so whatever, he didn't know.
This is a user-to-user forum. Read the ToS carefully. You and her want to have a laugh at his expense, that's fine. But users are expected to post on this forum to offer real solutions to other user's problems. At least mikeab shares a chuckle +along with+ a helpful word to his posters most of the time...
Now on with the show:
staffo, try replacing your iPhone's included SIM back into its tray, then dock it with iTunes. When iTunes comes up, select your iPhone device on the left side, and in the Summary tab, click the "Restore" button to restore it back to the original factory settings. If you have synced the phone before, then the restore will also sync up your old information. Update us on what happens, and welcome to the Apple discussion boards. =)