iPhone Unlock Only Limited To These Baseband

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Software (and hardware) unlocks

Ultrasn0w unlocks the following basebands (and ONLY these basebands)

iPhone 3G and 3GS: 04.26.08, 05.11.07, 05.12.01, 05.13.04, and 06.15.00

iPhone4: 01.59.00 (Gevey sim will unlock basebands 2.10.xx, 3.10.xx and 4.10.XX, but not 4.11.xx)

iPhone 4S: none *

*Back in November, 2011, MuscleNerd mentioned that a software unlock for the iPhone 4S on firmware 5.0 with baseband 1.0.11 (and only this baseband was mentioned) "might" be possible. No guarantee and no ETA. 

Unless your phone has one of the listed basebands, it CANNOT be software or hardware unlocked and there are no further software unlocks anticipated at this time (except as mentioned for 1.0.11)
There has not been a suitable exploit on which to build an unlock for almost 2 years, 7 basebands have been released which have not been exploitable for a software unlock.

Except for some very old 3G iPhones, basebands cannot be downgraded !
Downgrading the firmware does not downgrade the baseband !

If you require an unlock to use an iPhone, instead of leaving the phone sitting for months and months, consider selling it and buying a Factory Unlocked iPhone or buy one with the above basebands which can be software or hardware unlocked.
If you own an iPhone that cannot be unlocked, the longer you keep it, the older it gets and the less it is worth on re-sale market.
So, keeping it lying around in some faint hope of a software unlock breakthrough is not a logical choice.