I was playing with tether and ran the battery down very low. My droid is now in a reboot loop. I see the M, I see the charging light come on, I see the boot animation start, then I see the charging light blink 5 times and go off, followed by a reboot.
I tried restoring from a backup on the off-chance that it was software related, but it made no difference.
On a normal phone, when this happens, you have to let it charge before it will boot up - but the droid won't charge UNLESS it's booted up. Kind of a catch-22. I was able (just now) to get it to boot up all the way, and it says "connect your charger" (but it IS connected already!)