> a hand-painted cel is art, whereas a digitally
> painted one isn't.

Uh? A digitally painted cel ? You mean a computer 
file ? I don't think that you can call it a cel...

> Or that it somehow cheapens the image itself.

Err... Cels have a collector value because they 
are rare, because they have an "history" and 
because there was work involved in creating them.
I don't think that people would collect "jpg" 
files in the same way... :)

Cheers,
Yann Stettler