This place can get the sketch off for you, safely 
and without harm to either the cel or the 
sketch.  The man told me it takes about an hour 
to do, and two weeks or so to get the cel back to 
you.  I will be sending one here soon that also 
has the sketch stuck.  Im not sure what the cost 
is, but I think it's pretty reasonable, not to 
mention worth it to avoid damage to a lovely 
cel :).

http://www.srlabs.com/index.html