Ugo Cei: What's the point of having extensions in URLs? I use it to determine the desired format. You can see this blog entry (including comments) in html, rss, rdf, and txt formats. The rss2 feed is experimental.
Other aspects of the namespace: if the path element is exclusively digits, then it is interpreted successively as a year, month, or day. If it is alphabetic without an extension, then it is treated as a (as of yet unexploited functionallity) category.
Exploiting categories is on my todo list. As is creating permalinks for comments that people have left, and to integrate in a traceback system.
Oh, and queries are supported. And all of the above can be combined, so you can get a list of all of the postings on Keith in August of 2002 in RDF format, if you so desire. Filtered by category, once I start using that function.
All of this from a simple URL.