Freerunner won't boot: dies in uboot

[Chris Wright]

My Freerunner was acting fine until yesterday. Now, it refuses to boot
while not connected to a USB charger -- the power button does nothing.
When connected, it starts booting, but stops and turns off about five
seconds after showing the splash screen or uboot menu.

This happens when I boot into the NAND or the NOR bootloader. It
happens if I wait at the menu. If I go to the menu, I can start
booting the kernel, and I get as far as "Starting kernel..." and the
screen turning white (depending on how fast I am with the menu) before
the phone turns itself off.

I can do this many times, so I don't think it's a battery issue (also
since the bootloader usually detects whether it's connected to power
and starts charging, so I can just chill at the boot menu for an hour
and then boot like normal -- that's not happening now). I don't have
the wall plug, though, so I can't try that.

I've browsed the mailing list archives for something like this and
haven't found anything. Any suggestions on how to determine what is
wrong?