I installed Windows 7 on Boot Camp on my new iMac 27'' (ATI videocard) and everything was good until recently I noticed that the default Windows 7 background (then one on the background on login) looked jerky.
When I say jerky I mean the kind of jerky you get if you can't display enough colours, and instead of nice fading shades you just get stripes and jerky patterns. I am on native resolution but even if I go down to 1920x1080 I get the same.
This might have happened after a firmware update but as I don't use Windows very often I am not too sure it's what caused it.
Oh, and when I am playing games everything looks OK (as in not jerky!).