I had the problem this weekend. It went dead while sitting around idle. When I tried to charge using the stock AC charger it would just flash the charging symbols and not turn on. Eventually I plugged it into a USB port on my PC and let it sit for a couple of hours. It still flashed the symbols and wouldn't turn on.... but then I plugged it into my AC charger again and it turned on and charged up.
It was almost like when completely dead, the AC charger won't trigger a charge, only the USB trickle can get it to a level that will then allow the AC charger to charge.