We've got it fresh from the people who engineered this piece, people; hacker Zappaz and his crew have released an early, dirty, uncompiled version of their proof of concept iPhone SIM unlock source code. Hit the link and check it out -- we just suggest that unless you're a serious hacker you not use it yourself (yet) until the kinks are worked out.
Update: iPhone Dev Wiki is reporting that another compiled unlock solution has appeared, called iUnlock, developed by the iPhone Dev Team with credit to Zappaz. We've yet to verify that either solution works, but the iUnlock software appears to be in a more complete state. More up here.