I have the same problem with my iPhone 5. I've been trying to get past the "Unlocking... please wait" screen for half an hour now. Tried restarting, taking the SIM card out and putting it back in, etcetera. Connected to the Mac, not connected, doesn't matter. Even took the SIM card out while the iPhone was on, it said no SIM card was installed, then when I put it back in, the Sim Lock message came back. But I still can't get past the unlock screen.
Meanwhile I've upgraded to iOS 6.1, but that did not change anything.
I also tried giving it an incorrect pin just to see what would happen, but even then I'm not getting any reply, just "Unlocking... please wait". And on the next attempt I still have 3 tries left.
I can get out of the "please wait" screen by using the off-button, then on again, and selecting "Cancel" on the numeric keypad, but the message "Simlock active" comes back after 10 seconds or so. Even if I simply dismiss it with, it keeps coming back. And of course there's no network, just "Simlock". The phone does work as an iPod, though.
In the Telephone settings, Sim Pin code, everything is light gray with the "sim pin code" switch off ("0")?! It seems like the phone is having trouble communicating with the sim card? Both the phone and the SIM card are only three weeks old. Worked at one time, but then I did not use it for a few weeks and the battery had drained all the way.
Any ideas, anyone?
First of all, I'm not with AT&T but with Orange in France.
Second, I did not enter three wrong pin codes. In fact, whenever I get to the pin entry screen, it tells me I still have three tries left. I mentioned having entered a wrong code once, as an experiment to see what kind of reaction that would give after having tried many other tricks that did not work. But that's definitely not the reason I'm stuck on the unlock screen. It already got stuck before I ever tried an incorrect pin.
I did manage to get past the unlock screen once just a few minutes ago, but then the phone could not find a network and the SIM-related settings were still grayed out. I then restarted yet again, and got stuck on the unlock screen yet again.