had the same thing in 3DMark03 today. When running the benchmark it ran 90 degrees tilted. The settings also said 1024x1280 instead of 1280x1024. I also have a mac mini.
As a longtime windows user, I have seen this before, but bear in mind I'm not entirely sure this is what's happening to you.
Windows XP has a function that allows you to essentially flip the screen any way you would like, the purpose of which is incase you have an LCD monitor and would like to mount it on a cieling or wall somewhere. One day while trying to hit control alt delete I hit "control/alt/down" by accident... whoo boy. It flipped my screen upside down! Imagine the hilarity of me trying to read internet articles upside down trying to figure out just what the f### happened to my computer.
I'm not sure if it applies to flipping the screen left or right, but I can't see why not. if it happens again try hitting "control/alt/left" or "control/alt/right". Good luck!