Yes, the headbobbing is part of the reason why people get motion sickness. In fact, it is very unrealistic to actually see the head going up and down - normally, even though a human head does go up and down, it is balanced out so the sight does not actually percieve it as headbobbing. To be honest, I don't understand why so many fps games have this "feature", it doesn't make any sense at all.

Thank you so much for this :) Maybe this means I can play S.T.A.L.K.E.R after all :)