I've been using Thinkpads as full-time Linux machines for several years now. Specifically, I've had an R61, T60, X60, T530, X301, and W520. Aside from a rough start getting the X301 set up the way I wanted it, I've been totally satisfied. The Thinkpads I've used have all been sturdy, straightforward machines, easy to upgrade, and well supported by Linux drivers.

I'm currently using the W520 as my everyday development machine. It's heavy enough that I leave it in its dock most of the time, but taking it to and from work isn't terrible. It might grow annoying if you had to carry it long distances in a backpack or briefcase. I can't tell you what battery life is like because I'm either hammering the hell out of the CPU or barely touching it, depending on what I'm doing, so lifetime varies widely.

The X301 is much slimmer and lighter. Its battery life pretty much sucks, so I swapped the DVD player out for a second battery, and between the two it'll stand 4+ hours of heavy use. I use this machine as my personal travel laptop, and it is also the machine I use for DJ gigs.