I have an unboxing youtube video (posted in the custom hardware section) where I stopped in the middle of the video because I had the same thing that you have.  It was very easy to wipe off.  Just do it carefully.  I used a lot tissues and for the corners of those screws, i used cotton swab tips with rotating motion.  Not bad at all, and I was being careful not to put too much pressure on the circuits or scratch them.

Even after all of that, it's worth it as the latest Ant S1 is giving me 2 GH/s faster performance than my older Ant S1 (from GB5) that didn't have thermal pastes, both OC'ed.