Here's a discussion by some experienced curators over the framing issue:
http://www.anime-beta.com/phpBB/viewtopic.php?t=27461
The gist is that framing is tricky, unless the person doing the framing is familiar with the issues of possible damage from allowing the cel to stick to a backing mat or exposing it to destructive levels of heat/humidity. There is a way to frame and preserve cels in a museum-quality archival way, but it's likely to be expensive. Taking it to your local art store and having it framed like a poster or diploma is likely to be a recipe for short-term damage.
In general, yes, light from any source (sun, lamps, especially fluorescent bulbs) will slowly damage cels. This is because the materials are unstable and tend to react with each other, with light being a catalyst. However, short-term scanning and viewing is tolerable. My advice would be to keep them in a good archival-quality portfolio (Itoya Profolios are inexpensive and stocked by many art stores) and to look at them when you want to see them.
Sadly, when cels are damaged by light, it's not the light used when you look at them and admire them, but rather the hours and hours and hours of light they have to absorb when nobody is looking at them. So really storing them in a dark place or portfolio isn't ignoring them; it's saving them up for precious moments.
