Hm, I don't understand this. I could show you that there is even in full screen mode a flickering that disappears when I switch v-sync on in the direct-x properties.
So I am quite sure that v-sync is not always switched on in full-screen mode.
Maybe this is an issue of only a few users. But I have this problem with both of my 2 pc's with ATI Radeon 9500 and Geforce 6800 GT.