[hrjet]

Slightly off topic, but do you know whether the paid developers of Linux work on the core kernel, or on device drivers?

[abrahamsen]

Both, as they provide the bulk of the code. It would be more illuminating to examine where the unpaid volunteers contribute. My guess would be device drivers, but I don't know.

[rwmj]

This has some information although not exactly what you wanted to know:

http://lwn.net/Articles/579081/