Just unplugged my macbook from its power and initially the battery said 4:01 lef, however went to around 3 hours, thats in OSX with msn, firfox with multiple tabs, itunes, etc all running, ohh and airport, bluetooth etc all running
to be honest, don't even consider running windows on a macbook, thats just evil, i bought my macbook thinking i will install parallels and probably spend most of my time in XP world rather than osx as it was what i am so used to, thankfully i have seen the light and run osx full time now.
also, the macbook is very good at allowing you to alter all sorts of power saving rubbish so battery life can probably be increased with say shorter time to dim the screen and various other tweaks