I purchased a 32GB 3Gs back in June when they first came out and jailbroke it right away. It worked flawlessly until about a month ago when all of a sudden it wouldn't take a charge. I'd leave it on the charger all night only to wake up to find it drained even lower. After playing around with it for a while I discovered that if I turned off Wifi and the display I'd get it to charge back up. Something was still wrong as the charge indicator would fluctuate it's readings to below 20%, and then up to 60%+, and then down again. I ended up letting the phone drain completely to the point of dying and then tried to charge it again. This process seemed to work as the phone was able to charge up to 100% and it started to behave normally again.
Then the other day I noticed the phone wasn't charging again. This time I thought I'd be smart and just drain it down and then I'd be back in business. Well, this time that didn't work. The phone would acknowledge being plugged into the charger, but no charging would take place. It was stuck on the empty battery with a small red line indicating that the battery was empty. Wall charger and USB charger on the PC didn't make a difference. The PC wouldn't even acknowledge that the iPhone was plugged into it.
I tried doing hard resets, but that didn't work at first. After trying a dozen times or so I finally got the phone to a point where iTunes would recognize it, but it would need to be restored. I ended up having to restore as new with the 3.12 update (had been on 3.0 previously).
I thought I could downgrade to 3.0 since my ID was on file with Cydia, but I couldn't get that to work after several attempts. Even though I followed the instructions to alter the HOSTS file to have Cydia pretend to be Apple's server it would never work. I decided to just stick with 3.12 since I knew I had an older 3GS and would just use blackra1n to jailbreak again. That all worked fine and dandy, but the downer was that it would not accept my previously saved backup since it was done with 3.0. The good news is that the phone was now charging back up to 100%, but I lost all my info since the backup wouldn't take.
I ended up restoring the phone again to new and returned it to the Apple store. They swapped it out for a new one due to the charging issues. My only real concern was that they would give me a newer 3Gs phone that would only be able to have a tethered jailbreak. Luckily, no. It must have been sitting in their stock for a while as it works great and I don't have to re-jailbreak it each time I turn it off or reboot.
I'm just pissed that I lost my contact info. Some info I'll never get back, but fortunately it's just a matter of taking the time to type everything else back in.