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 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."
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.
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.