I don't know where this guy is based, but his IBM email address is @linux.ibm.com, so arguably his work on the Linux kernel may be conceived as "relating to the employer's business."
Delaware 19 Del.C. s 805
Illinois 765 I.L.C.S. 1060/2
Kansas K.S.A. 44-130
Minnesota M.S.A. 181.78
North Carolina G.S. s 66-57.1
Washington RCWA 49.44.140, 150
Utah UT ST s 34-39-1, 2, 3
IMO this is the gray area. Could IBM make a reasonable case that this kernel work is related to / close to / similar to what IBM is doing? Could they have learned something, even just a rejected -- but proprietary -- approach from an internal project... that kinda ends up on the kernel?
I agree with the consensus that IBM is in the wrong here... but lawyers might be able to prove otherwise.
"
2870 (a) Any provision in an employment agreement stating that an employee should assign any of his rights in an invention to his employer shall not apply to an invention which the employee develops totally on his time without using the employer's equipment, supplies, facilities, or trade secret information except for the inventions which:
2871 Relate at the time of conception or reduction to practice of the invention to the employer's business, or actual or anticipated research or development of the employer
2872 Result from any work done by the employee for the employer.
"
In the state of California clauses assigning _all_ IP to the employer in an employment contract are not legal, and hence unenforceable.