I get only about 2.5 hours when running Arch Linux compared to almost double when I used to run OS X on my MBP 5,3. I rarely use my laptop unplugged though so it doesn't make a huge difference to me.

I think a large part of the difference on my machine is that you are forced to use the 9600GT for video as opposed to the more efficient 9600M that is available in OS X.