Because I just had a minor panic thinking that the screen on my MacBook was completely screwed, and I think the solution needs a bit of Googlejuice:
The symptom: Your screen shows patches or bands of alternate in-focus and out-of-focus pixels, as if it was set to a resolution one or two pixels out from the correct one. Moving your mouse pointer up and down the screen causes the screen to shift up or down by one pixel (and the blurriness to move to another region). Restarting doesn’t fix it, although the problem only reappears part way through logging in, and it doesn’t affect any other users on the system.
The solution: Press Alt+Apple+8.
The explanation: You accidentally turned on OS X’s ‘zoom’ accessibility feature. It seems that if you’ve never used this before, or you have your preferences set up in just the wrong way, the default zoom level is “just a couple of pixels in from full screen” – enough to make things go nasty and anti-aliased without it being obvious that it’s zooming in on anything.
…and no, this is not your cue to crack jokes about things that give you blurry eyesight and their possible correlation to being a mac owner.