I trun off my iBook every night. It gets really hot, even when it's sleeping, so I give it a break.
Call me overcautious, but I don't think it helps the computers life span if it's left on all the time, but maybe that's just me.
Actually, turning computers off and on produces more stress on it than just simply leave it on. This is due to the components expanding and contracting from heat, among other things. Either way, you'd be upgraded to something newer by the time any problems started to show anyway, so I wouldnt worry about it.
EDIT: Didnt notice how old the thread was.