Become a MacRumors Supporter for $50/year with no ads, ability to filter front page stories, and private forums.
It allows you to load your own custom firmware which is lower than your phones current firmware (3.0.1 to 3.1.2 why not?) though I believe you need your phone to be on the lower baseband (someone correct me if I'm wrong...)
 
download fuzzyband from cydia and this will let you know if you can downgrade the baseband or not and if you can it will do it for you I believe
 
download fuzzyband from cydia and this will let you know if you can downgrade the baseband or not and if you can it will do it for you I believe

FuzzyBand only works for about 10% of very early 3G phones (must have bootloader 5.08).

It won't work on the rest of 3G or any 3GS phones.

3G phone firmware can already be upgraded and downgraded at will (except the baseband of course)
 
iREB is iTunes Restore Error Blocker. Basically eliminates the 1600 error you get from custom firmwares and downgrading firmwares. It will not downgrade basebands. The newer versions of iReb have more tools integrated into them, like Hash grabber and boot loop bypasser. iH8sn0w hasn't finished all the tools in it though, as he is busy working on sn0wbreeze at the moment.
 
Register on MacRumors! This sidebar will go away, and you'll see fewer ads.