I bought this adhesive remover from my local art 
supply store called Un-Du.  I have taken several 
drawings off this way, about five.  Their was 
only one i had a problem with, the drawing was 
ruinged, but every other time i kept the drawing 
and the cel.  Wait, one time some paint did come 
off, like a blurry outline, no major paint 
flakes, it was my first attempt on a cel that i 
didn't consider very valuable.

i am pretty sure there are a few sites that also 
have information about other processes to loosen 
drawings from cels, but i can't remember where 
those site are.