I had the same issue today that others have mentioned already - battery died and would not charge at all. I pulled the battery out and placed it back in, but when i tried to charge, it would turn on for a sec showing a battery bar or whatever, then turn right off. I tried charging via USB connected to my computer and the official wall socket plug - neither worked.
Luckily, my co-worker has the same phone. I swapped batteries with him to see what the issue could be. My dead battery was able to charge on his phone and his battery (not dead) was unable to charge on my phone, but my phone did still turn on with his battery. I guess the issue has something to do with the charging connectors or something.
So in the end, I just charged my battery on his phone for a few minutes to just have enough juice to turn on my phone. Once my phone was on, my phone was able to charge the battery again.
Weird.