I have had this issue for quite a while, and have found no info to help me. Also, the issue doesn't always show up. What is happening is when I select something with the marquee tool, the selection doesn't start at the position I click. It's usually a few pixels off. It also does this with the ruler tool. I'll click at a point to start measuring and the ruler starts at a completely different spot. Really annoying! Anyone else have this problem and/or know a way to fix it. I'm using Photoshop CS4 btw. Thanks!
I've seen this before as well. I was able to fix it by disabling “Use Graphics Processor” under Preferences / Performance (I'm using CS6, so the names may be slightly different).
There's a little more discussion here: Photoshop: Inaccurate selections with Radeon graphics cards on Mac OS
Of course, disabling GPU will slow things down...
It's worth noting that I'm not seeing this issue now (CS6 / 10.7.3 / Mac Pro / ATI Radeon HD 5770).