I had this problem just now.
My Droid died after getting down below 5%. I'm at work right now, and only have a USB port to plug into on my ThinkPad T61. The one I normally use sits by itself on the right-hand side. Wouldn't turned back on plugged in. Tried reseating battery; nothing.
So I started playing around with it, trying different key combos and such suggested here. Nothing at all.
I switched to a different USB port, thinking the other one was lower power (you never know with a notebook). Tried key combos again, still nothing. I put it down, and a minute or so later, it just turned back on all by itself.
So it sounds like the wall charger idea is just because it's getting enough power to do the job. If you don't have the wall charger to work with, try a different USB port (if you're on a desktop, try the ones in back of your machine rather than ones on the front or an attached hub).