My own Bedside is designed to be an always-on, LED-style clock for just such a use:

http://forum.meego.com/showthread.php?t=4693

On a fully charged N900, it only went down to 50% when used all night. The next version (should be released this weekend) will have customisable colours and remember whether it was in dimmed mode (sets backlight to lowest and mutes display) when stopping/starting the app.