I keep the battery in. My computer tech friend suggested that it's a good idea to unplug your laptop -- about once a month -- and run it on battery power until it drains, then plug it back in and let it recharge fully. I have never removed the battery from my laptop.

I have had my iBook for nearly 3 years and I still get about 5 hours full power on battery alone. I don't know if it's because I do the monthly battery drain maintenance, but I'm happy with how my laptop's working, so I'm going to keep doing that.

I use a Toshiba laptop at work and I keep the battery in at all times too. All my colleagues do the same with theirs.