My work laptop (HP Compaq 6710b, 2.2GHz Core 2 Duo, 1GB memory) already gets three hours in Ubuntu Hardy versus two hours in Windows XP doing the same things (mostly Firefox 3). Mostly from ticklessness, I think. Others have noticed that Hardy (kernel 2.6.24) uses notably less power than even Gutsy (2.6.22).
(There's a "Windows Vista Basic" label on the laptop. I shudder to think how hot that would run. When it's on XP the ventilation port blows like a hairdryer.)
Powertop helps you use even less. It shows precisely what's sending your CPU into a high-power state, and makes helpful suggestions for simple stuff to use less power. ("Press B to switch off Bluetooth.") I should be able to manage 3½ hours easily.
I believe Powertop is in the default installation of Hardy. If not, enter sudo apt-get install powertop from the terminal. Then run it with sudo powertop and glance at it every now and then while you're on battery.
Even if you can't do anything useful with the results (it's really a programmer's tool), it's good for bragging matches with Winders devotees. GO GREEN — GO UBUNTU. Windows' carbon footprint is UNSUSTAINABLE.