That's pretty much 95% of the posts on UsesThis. Which I wrote off at first as being elitist, but the more top tier developers you meet the more you realize that it is actually the norm. The interesting thing is that for those not using a Apple laptop are always on a Lenovo.
I'm working on a infographic, or something along those lines, that shows a breakdown of what it used on UsesThis, but the pie charts are going to mostly be Apple flavored.
I assume the reason they're on a ThinkPad/Lenovo is that there is a well-deserved reputation there for running Linux well.
I recently picked up one of the new Toshiba Portege models to run Linux, and stock Ubuntu is giving me flashbacks to the circa-1999 Linux experience -- it doesn't detect when a monitor appears over HDMI, randomly will run up to 100% and freeze (kernel patch coming soon!), power management settings seem basically ignored with the display dimming when that is both disabled and I'm typing... and the worst part? Mentioned it to the local department Linux people, and they just said, "oh, yeah, you should've bought Thinkpad."
I just got a Thinkpad T420s and almost everything worked right out of the box (Ubuntu 10.04) despite some people having trouble on the forums a few months ago.
Sometimes I have problems getting the display connected to the docking station to detect if I suspend while not connected, then connect, then resume. This is a pretty minor flaw that is fairly easily worked around.
Before switching to Apple laptops, I had several Thinkpads - and would have to say their solidity, reliability and no-frills approach were major factors. I'd probably go Thinkpad again if I needed a non-mac machine.
Another factor represented here is they're popular in the corporate world - I know Adobe is standardized on them for the Windows side and I /think/ Google is too.
I'm working on a infographic, or something along those lines, that shows a breakdown of what it used on UsesThis, but the pie charts are going to mostly be Apple flavored.