We investigate analytically the spin-laser coupling of an electron in an ul
tra-intense linearly polarized plane laser field. Particular emphasis is pl
aced on the reaction of the spin on the electron motion which, in contrast
to a recently applied approach, is calculated by means of a quantum mechani
cal description. An additional oscillation in the direction of the magnetic
field is predicted and the classical results are confirmed up to a numeric
al constant. By comparison with Klein-Gordon results we show that this effe
ct is doubtless a signature of the spin, and isolate the leading second-ord
er correction terms in the ratio of electron and light velocity.