> ARE THEY SAFE!!!???

No.

> Does the sun fade cels?

Oh, lordy, does it ever.  I have learned (by hard 
experience) to NEVER, EVER buy a framed cel on 
Yahoo Japan.  Because the chances are as good as 
not, that the cel will have all of the lines 90% 
faded out from hanging on the wall and being 
faded by the Japanese sun.

> Can light-bulb light fade cels?

I don't know for sure.  My personal opinion is, 
probably not fast enough for your to care, 
because incandescent light bulbs emit in 
wavelenghts that are relatively low and non-
energetic.

> Do they just fade over time regardless?

Yes, positively.  There is a well-known positive 
correlation between line fading, and lines that 
are within or surrounding "warm" colored paint, 
such as brown and orange.  I am not aware of the 
chemical explanation for this effect.

> Does any kind of glass stop sunlight from 
fading cels?

Your guess is as god as mine...

> Does simply having cels is anything but a 
complete dark room have the potential to fade 
them?

There is some reason to believe that lines react 
with some colors of paint (such as brown and 
orange), so that they will fade even in a 
complete dark room.

> How fast do they fade? (I know...thats not very 
specific...;_;)

And the answer is not very specific.  It depends 
on how the lines were drawn (india ink, versus 
photocopier, and in the case of photocopier, 
which BRAND of photocopier), and also depends on 
the colors of the paint touching the lines, which 
company manufactured the paint touching the 
lines, maybe which production run that paint came 
from and which solvents the paint company could 
buy the cheapest for that production run, etc.

You puts down your money, and you takes your 
chances....