Nail polish remover is a no no on cels.  It will 
damage the acetate and often causes it to warp.

Paint thinnner (mineral spirits) sounds like a 
bad idea cause of the damage it could cause, but 
due to its strenght should clean the cel.  
Haven't tried it so can't comment one way or the 
other from experience.

Only solvent I've had any luck with is isopropyl 
alcohol, the 90% solution works pretty good on 
cleaning surface grime up.  And due to the nature 
of the cleaner it evaporates really quickly which 
is convenient.  To get really bad spots try 
wetting a q-tip or cotton ball and going over it 
a few times to clean the cel up.  Its the only 
thing I've found that can safely remove surface 
garbage from cels so far (including permanent 
ink).