Conservation glass is very important. :)  Make
sure whatever matting you use is also acid-free.
That may or may not be harder to find in a pre-cut
size, but it's not hard to get framing shops to
cut mats for you.  If you do go to a framing shop,
make sure you look through their in-stock acid 
free mats so you don't have to wait for the order
to come in! :)  I have 5 cels framed and they all
look wonderful.  All of them lay flat on a piece
of acid free mat or, in the case of one, the
production background and I have not noticed any
warping.  I think Drac's warping may be 
exacerbated by the spacer.  Pay heed to Drac's 
warning to have a deep frame!  Your 'sandwich' can
get quite thick. :)  Remember, even tho you
have conservation glass, don't hang the cel 
where something like a sunbeam will hit it.  UV
glass can only do so much.  
Have fun!

At the bottom, I am including a picture of one
of my framed cels.  This was done by a pro
art framer.

Jen