Recently, I started having an issue with my Macbook Pro where, when using the mouse/trackpad, everything on the screen was being selected. Not only was it preventing me from getting anything done, it was driving me crazy! Luckily, I discovered that others were having the same problem, and several solutions were offered on the Apple discussion forum.
I was happy that the simplest solution offered worked for me: “a quick double-tap” on the trackpad. Doing this will deselect everything and the trackpad will behave normally (until the next time it happens). This worked for me on Mac OS X (10.6.4). Hope it works for you as well!