Some brave souls have tried to update their SIM Unlocked iPhones to 1.1.1 and there appear to have been mixed results.
Jonathan Seff at iPhoneCentral gave it a shot and found that his unlocked iPhone now gives an "Incorrect SIM" error. He even went to an AT&T store who gave him a new SIM and still no luck.
Meanwhile, a few anecdotal reports indicate that some people have been able to successfully upgrade their unlocked iPhone to 1.1.1 without any ill consequences.
At this early stage, we'd recommend people with Unlocked iPhones continue to wait to update while more definitive testing has been done.