I've gotten a number of "script requests" asking for ways to filter out search keywords from the referrer. It's not a simple task. I did it once by hacking a script I found – and even then, I got it only to work with google. There's really no point in reinventing the wheel (especially THIS wheel because it's particularly complicated) so I went looking this morning for something that did this (My plan was to hack out the piece that I needed).
I found this page which has a really neat looking script for tracking (looks like it does some similar things that I did with mine that I'm holding off releasing).
Looks like a few other scripts by the same author are linked from there, too.