I've used the freezer method on backgrounds 
before. The trick is knowing which ones to try it 
with, and when it's worth risking. If a cel has 
totally and completely combined with the 
background paint over most of the cel, leave it 
alone... if it's partially stuck & you are 
worried about color bleed from the background, 
you can try... just don't try to YANK the cel 
from the background... I find sliding a piece of 
paper against the stuck again is pretty 
effective, and there's less of a change to pull 
the paint away from the cel.

TTYL

Karen