Today, i cannot imagine the internet without RSS. My subscription portfolio is currently in the range of a few hundred RSS feeds. For all us techies, here a brief on USM by Randy Charles Morin, KBCafe.com.
Here's a Initial extract:
Universal Subscription Mechanism is a really simple way of allowing RSS readers to subscribe to RSS feeds. Many blogs and Websites with RSS feeds present an orange XML or RSS button, Atom button or text anchor link that points to the RSS feed. When the user clicks on the button, nothing substantial happens. This mechanism replaces that click with a subscription notification to the users default RSS handler.
The mechanism defines two requirements of the RSS feed publisher; to return the application/rss+xml Content-Type in the HTTP headers and to place one new Atom Link Construct element in the RSS channel. The mechanism then allows RSS readers to create a simple shell extension to initiate the RSS feed subscription using a small native application.
Also, here are a list of clients supporting USM.