I don't think there is anything that will remove it. I remembered seeing this on Artics Silvers website just yesterday when I was looking over the products.
"CAUTION!
Arctic Silver Thermal Adhesive is a permanent adhesive.
If you do not follow the instructions for diluting the adhesive with Arctic Silver Thermal Compound, any components you attach together with full-strength Arctic Silver Thermal Adhesive will stay attached forever."
When you put it on did you mix it with the normal stuff? If not you could be out of luck.
If you did mix it right, you should be able to remove it normally. I remember reading an article (was it at hardocp? can't remember) where a guy did this exact same thing. It could not be removed with damaging the processor. Yikes.
Only thing I can suggest is nail polish removal gel. My wife has a bottle that I use to dissolve crazy glue and things like that almost instantly. I don't know if it will work on this stuff but you can give it a shot. Try mixing a little on something first and then use the gel on it. See if it works. If it does, you can start very carefully on the edges of the heatsink but I don't know how you will get to the stuff in the center. Good luck!
Dirk