Make sure your dock connector is not loose - I had an issue with mine and had it swapped out. The right side was loose and the USB cable wouldn't sit still - even the slightly movement would cause it not too stay in correctly.
It will look like it's charging but somehow it doesn't charge all the way through. I would start there.
I'm getting this too! I just plugged it in again and it seemed to charge okay but then started dropping off very quickly, I plugged it in again and wazzed it back up to 100% and it seems to be discharging as normal.
Am going to take it in and get it checked out when I get a chance.
Yea just unplugged and replugged to get 100% again...weird
Seems it is a common issue for 4S but not my iPhone 4 though (I have been using that for one year without this issue)
What do you think guys? is it a hardware or IOS 5 related issue?