I got a note from Michael of seo-blog about and experiment he made which I find really interesting. It was an experiment to determine if Google, Yahoo and MSN index hyphenated, underscored and conjoined keywords in URLs.

The results were as follows:

Google indexes on keywords in hyphenated urls but not on keywords in underscored or conjoined urls.

Yahoo indexes on keywords in hyphenated and underscored urls but not keywords in conjoined urls.

MSN indexes on some keywords in hyphenated, underscored and conjoined urls but the exact circumstances in which it does so are unclear.

The original experiment and conclusions are here http://www.seo-blog.com/keywords-in-urls.php and the follow up is here http://www.seo-blog.com/url-update.php