According to the recent tweet by @MuscleNerd, Gevey interposer hole has been closed by Apple in iOS 5 beta 2. This was the only method after ultrasn0w that allowed user to unlock their iPhone 4 on 2.10,3.10,4.10 BaseBand until Apple seeded iOS 5 beta 2 to developers yesterday. Generally, registered Apple developers doesn’t require an unlock but it seems that the loophole has been closed prior to the final release. It is assumed that the closure of Gevey Sim method is due to some kind of pressure.
Those iPhone 4 users who are on BaseBand 1.59 will be able to remain preserve their BaseBand in future as per the confirmation from @notcom, TinyUmbrella developer. Perhaps, the functionality has not been implemented yet. But we may be able to see it in the near future. For me, it is not something new as methods similar to Gevey sim have been closed before. One of the example is the turbo-sim which was fixed in OS 2.x.
It you want to know about the Gevey sim method, check it out from here