People who changed their battery to a unofficial replacing battery might get problems with updating their iPhone 3GS to iOS 4.
If there's placed a new battery, bought at eBay, Amazon or iPhone reparation service, the iPhone might get stuck in iTunes with Error 29.
After that the iPhone stays in recovery mode. Placing back the original battery isn't an option for many users. There isn't yet an solution. The problem doens't exist at the iPhone 3G and the the 1st generation iPhone. If you have this problem and already updated to iOS 4, you can get instructions from milliam to downgrade to 3.1.3 (click here to get the instructions)