My biggest headache now is find a iPhone 3Gs with iOS 3.1.2 on it. iPhone 3Gs is everywhere, but all of them are with iOS 4.1 or even higher.
My question is, is it possible that I get a iPhone 3Gs (with iOS 4.x) and some software package so that I can restore it back to iOS 3.1.2?
Btw by saying 3.1.2 I mean the official non-jailbreaking 3.1.2.
Many thanks,
Di
You would have to have your SHSH's saved with Cydia, then you can downgrade to any version you like (that's supported). With that said, after downgrading, You could always not jailbreak, so it's a legit iOS 3.1.2, but you would have to be jailbroken at one point prior. Other than this, NO, there is NO WAY to DOWNGRADE from any iOS version.
Alternatively, You can try Xcode to downgrade your device as a Developer Device. This would probably be the closest I can think of that Apple would let you downgrade. But it is a longshot.
Just in case you meant 3.1.3 (not 3.1.2), there is this article from lifehacker on how to downgrade from iOS4 to 3.1.3: http://lifehacker.com/5572003/how-to-downgrade-your-iphone-3g%5Bs%5D-from-ios-4-to-ios-313. I used this successfully to downgrade to 3.1.3.
Based on the article, it looks like you could do the same in downgrading to 3.1.2, you just need to find the right image for it.
来源：https://stackoverflow.com/questions/4487314/a-timemachine-taking-my-idevice-back-to-3-1-2