Actually, the cost to distribute each copy is not almost nil. Most publishers aren't handling this themselves. Why? The demand simply isn't there to bring on the necessary staff, additional equipment, and write the necessary software themselves.
Instead, they hire out a firm, like Lightning Source, who are charging fees to handle conversion and distribution that are in the same neighborhood as conventional printing and distribution per copy. The publishers, to a large degree, have not realized the savings yet as they haven't even built the infrastructure yet.
The Kindle is far too new to have had an influence on prices yet, but I believe that is coming. And if the fuel crisis was/is prolonged, you will see it influence the price of everything that entails physical distribution, including pbooks; it takes time for the reports to show the real impact and prices to adjust accordingly.
I have no doubt that ebooks will eventually become cheaper than pbooks, but this will take time. The Kindle--love it or hate it--is going to do a lot to speed the process up, but it will still take time. I have my doubts that this will be realized as a price drop of ebooks rather than a price increase of pbooks though due to economic and environmental influences.