i took a black macbook at the london apple store and unplugged the power, airport was on and it was idling and it reported 2:11, which is piss poor i just unplugged my 4 year old 600MHz ibook which is in much the same state and it now says 3:06 remaining.
I'm getting about 3:30-4:00 from mine with brightness about 75%, wifi on and bluetooth off and using safari, iterm, mail, iTunes, Word, skype, TextMate, etc...
It's about the same as I get from my 17" MacBook Pro.