Ugo Cei has been having a small dialog on his blog about using extensions in URLs.  I've recently be contemplating the same thing because we discussed it in my class last Monday when we were talking about Tim Benners-Lee's "Cool URIs Don't Change."  Ugo says:

What's the point of having extensions in URLs? What if someday you adopt a system whereby you can serve your content in different formats (HTML, WML, RSS, PDF, etc.) to different devices or users based on the User-Agent HTTP header? And all from the same URL? Would it still make sense to use the .html extension?

I've personally gone through the hell of changing from .html to .shtml to .jsp and then back to .shtml as I shifted technologies on a web site and it would have been much nicer to have had no extensions to deal with.  This document discusses content negotiation (which is how you make this work) in Apache.