Hold down PMT while turning on the unit. Options include "Tst Screen", "Tst Keyboard", "Checksum", "Tst Battery", "SW Version", "(c) Copyright 2008 HP Dev. C., L.P.", and "TSys Off".
TSys can be toggled on or off. Not sure what it does. Screen and keyboard test are pretty much what you'd expect. Mine shows a checksum of 213.213, battery of 3.500, and the software version is 6 24 2008 (tuesday) or 6.242.
Hit the input key while in battery test, and the display will switch to a live measurement. I ran it down to 2.1V before the readings became inaccurate. (It shut down at about 1.8V or so.)
Looks like the reset all key combo is hold down N and Amort while powering up.