"There is no way to come back down from 06.15, and there’s no hiding the baseband version from Apple. You’ll be voiding your warranty in a very obvious way.
If some future baseband comes out with a critical fix, you won’t be able to update to it if it remains down in the 05.xx sequence (then again, you wouldn’t update to it if you wanted to keep your unlock anyway).
Starting with FW 4.2.1 if you have 06.15 on your iPhone you won’t ever be able to restore to stock firmware (it will fail). You’ll need to only restore to custom IPSWs (then again, if you’re unlocker you should already be doing that).
Unlockers have been reporting mixed results about GPS functionality at 06.15.00. Until we can track down what differentiates those who retain GPS vs. those who lose it, be conservative and assume you’ll lose GPS at 06.15.00. As we work on finding the cause (and possibly a fix), please report your personal findings in our comments section. (Update: early indications are that while 06.15.00 is capable of GPS, it will require some further hacks. But please still be conservative and assume you will lose GPS at 06.15, in case the hacks don’t work).

Certainly don’t update to 06.15 if you don’t need to! Only do this if you need the unlock and you’re stuck on 05.14 or 05.15, and you’re willing to assume the above risks."

Forrás: http://blog.iphone-dev.org/post/1718400992/ultra-recycle