I just bought a ThinkPad T510. What caused me to convert was the advertised EIGHT HOUR battery life. Yes, it's true. Under moderate load you can expect to get maybe six or seven; under heavy load maybe four. But that's not bad. Better than many netbooks and this is a full-sized laptop.
One data point: running Ubuntu on my T510 I don't get anywhere near eight hours with the standard battery - at least not with wifi on. However, I haven't done any battery life tweaking (powertop etc), and it probably doesn't help that I have the higher-res 1920x1080 screen (which I assume chews up battery, but I wouldn't give up for anything).
However, I got the backup battery as well (it plugs in over the top of the normal battery) - it pretty much triples the battery life. It's heavy as hell, but if you're going to be in one place for a while (at home, on a long-haul flight, etc) that's not a problem.
Arch Linux, no desktop (just a bare window manager with xterms, emacs, and stuff), and I turned the screen brightness down to the lowest comfortable level.
