What is USM?
I installed the USM Windows Reference Installer from this page and chose My Yahoo! as my RSS reader. Now I can click feed:// URLs (like this one) and they get added automatically. That’s pretty cool because if everyone adopts this, then blogs only have to provide one feed:// URL instead of providing a zillion dinky buttons for add to My Yahoo!, add to bloglines, etc….
More info…
The USM client works with the thousand or so blogs that support the feed: URI scheme. It also works w/ the 1-2-3 million blogs that support the USM protocol. Typepad support USM out-of-the-box. In this case, the server still uses the http: URI scheme but returns the application/rss+xml Content-Type in the HTTP headers. For an example, check The RSS Blog (http://www.kbcafe.com/rss/) and click the little XML/RSS chicklet.