PC Mag took a look at a bunch of ultra mobile notebooks:
Typically, when it comes to battery life, you want something that'll last you through a work day, or at the very least, a coast-to-coast flight. The Lenovo Thinkpad X60s gets you over seven hours of battery life using the extended battery. The HP Compaq nc2400 and the Gateway E-100M ship with standard 3 cell batteries that get about 3 hours of battery life. Both have options for larger batteries that cost minimal and will let your system last at least seven hours.
There's more to the extended battery life than the battery though. Low powered components help, too. And low powered components equal mediocre performance..