same problem here too
I have the same problem too. I leave the phone (iPhone 3G) plugged into my computer using the provided sync cable for hours. The whole time, the charging indicator will be on, but when I disconnect, I'm down to little (10%) or no battery (shuts off and displays the battery icon).
Really frustrating -- didn't want to waste ANOTHER day at the apple store, and having dropped a whole bunch of cash (actually credit -- the Apple store doesn't take cash!?!) this weekend to buy the thing, really didn't expect to have to trek back there.
I want to love the phone, but combined with the fact that my existing music docks don't work with the thing, the underwhelming 3G coverage area, terrible battery life, etc.), I'm having a tough time. I'll work on it, though -- hope Apple will work with me