I've used acetone for cleaning pcb boards for years, I've used it in client systems when removing thermal paste, I use it to clean my thermal paste off of my own parts ranging from video cards to cpus. Hell, I just used it on my video card pcb when I accidentally had some paste on my finger during a heatsink re-seat and smeared on to my 7200gs when handling it. I buy pure acetone from my local hardware store, no additives.
Just as a safety reminder, for both acetone and alcohol chemicals. Please work in ventilated areas and if you touch either one a lot (ie: daily), please wear chem resist gloves.