Ok, I've had a further look, removing the bq27x00_battery module gets rid of the extra voltage reading that is messing things up.

EDIT: My N900 has had 2 random reboots, each time after removing that module, not immediately however, it could be unrelated, maybe something else due to the new kernel-power, but I'm not overclocking and my N900 has been otherwise stable before installing kernel-power. So just a warning it case it is related.