Just to preface this before anyone gets the wrong idea; there is still no new baseband unlock available.
Now that the formalities are out of the way, I think it's safe to tell you that new details have been revealed about the existing baseband unlock and its compatibility with the soon to be released untethered jailbreak for iOS 4.3.1.
Depending on when the jailbreak is actually released, if you rely on an unlock, you may have to wait a while until a new version of UltraSn0w is released. This is due to some compatibility issues between iOS 4.3.1 and the current version of UltraSn0w.
Fortunately, you won't have to wait long, as the latest tweets from members of the iPhone Dev Team confirm...