MuscleNert in a couple of his recent tweets mentioned that downgrade for iPhone 3GS/3G 06.15.00 band to any older one would be released pretty soon.
The jailbreak and unlock tech junkie recommends that it would be better to downgrade it to a baseband like 05.13.04 where both the unlock and the GPS are working fine according to his version.
The most impressive thing about unlocking iPhones is that you get to use your own service provider and be free from the need to be locked up in a contract. However, software unlocking also has some issues and problems associated with it.
But before we jump into that, here are the tweets:
Made great progress yesterday on downgrade from BB 06.15 (for 3G/3GS owners who got officially unlocked). All the pieces are there
@eriksmets Yep! This lets you downgrade 3G/3GS from 06.15.00 to any prior BB (05.13.04 would be good pick: ultrasn0w + GPS both)
There’s not a big difference between a factory unlocked iPhone and a software-unlocked iPhone. However, with a factory unlocked iPhone, which costs a lot actually, you will always be able to upgrade to the latest iOS without any trouble.
With a software-unlocked iPhone, you can only get the iOS version for which there is both a jailbreak and an unlock method available. This means you cannot always have the latest iOS until unless there’s a jailbreak and unlock available for it.
Currently, Ultrasn0w can unlock iOS 5.1.1 on iPhone 4/3GS with support for up to five basebands.