A while back I saw some independent tests that showed keywords in URLs definitely counted in some SE results (Google and Yahoo included). Plus if you read some of the comments from Google themselves then seems to suggest there's some benefit:
From Sitepoint: "What Is the URL structure preferred by Google?
Google’s Matt Cuts replied:
I would recommend
long-haired-dogs.html
long_haired_dogs.html
longhaireddogs.html
in that order. If your site is already live on the web, it’s probably not worth going back to change from one method to another, but if you’re just starting a new site, I’d probably choose the URLs in that order of preference. I can only speak for Google; you’ll need to run your own tests to see what works best with Microsoft, Yahoo, and Ask."
However, I think Ryan's comments are spot-on regarding the file extension part; i.e. has no effect other than pages already ranked by Google from an old site which include specific file extension
Rgds M