My understanding is that it is usually the paper, more than the pigment in the pastels that deteriorate due to light. Some colors are more lightfast than others (yellow is usually a problem), but this is true with paints as well as pastels.
I agree that the support used for a pastel work is the determining factor for longevity. If you use an actual paper product, clearly it will become subject to the ravages of time, but some of the sanded "papers" are rated at 300 to 500 years (e.g. Wallis brand). Then of course, there are pastel works on panels. I would venture to guess that the exquisite works of Sharon Knettel will survive to be admired for hundreds of years.
The real trick to ensuring the longevity of a pastel work is to make certain that it is properly framed under glass using archival quality (acid free) materials. Articles that I have read make claims that works of pastel on paper retain a more brilliant color than oil paintings of a similar age because of the yellowing of many varnishes, etc. I think, in the end, it all comes down to personal preference and feelings.
I am sure delighted that we have outstanding artists out there who get the same prices for their oils and pastels. I am personally much better with pastels than oils - the opposite issue from Mike. Maybe I should charge more for pastels?
Great thread, interesting discussion!