i had one of my swan princess cels come like 
that... i was so mad for the longest time ^^;

but what i did was, i put it in my cel book and 
left it there for a month or so... when i took it 
out, the cel was flat :)

if you don't have a cel book, you could always 
put the cel between two flat boards and leave it 
that way for a while...

wel... hope that helps. best of luck with your 
cel :)

~ryoendymion