Not to say it's actually easy to get your hands on the rumored 1.1.3 firmware at this point, but in the event that you do, proceed with caution. Nate True, the guy that brought us a video walkthrough of the supposedly leaked goods, is reporting that he can't seem to downgrade to 1.1.1 or 1.1.2 after performing the install. Locked phones show an "Incorrect SIM" error -- a problem unlockers of yore are all too familiar with -- while previously unlocked phones can't be upgraded to 1.1.3 at all. Let's be honest, though: if you somehow managed to score a copy of 1.1.3, is that really going to stop you from going for it?
Update: We're hearing that the phenomenon observed by Mr. True has existed since the very first firmware release; it's a state the phone enters whenever it finds a baseband version different than the one it's expecting to find, and it's apparently been bypassable via a modified "lockdownd" binary for some time. Thanks, Alex!