I've written the first draft of the specification for the "feed" URI scheme. From the abstract

This document specifies the "feed" URI (Uniform Resource Identifier) scheme for identifying data feeds used for syndicating news or other content from an information source such as a weblog or news website. In practice, such data feeds will most likely be XML documents containing a series of news items representing updated information from a particular news source.

The purpose of this scheme is to enable one click subscription to syndication feeds in a straightforward, easy to implement and cross platform manner. Support for one click subscription using the "feed" URI scheme is currently supported by NetNewsWire, Shrook, SharpReader and RSS Bandit. The author of NewsGator has indicated that support for one click subscription using the "feed" URI scheme will exist in next version.

Any feedback on the draft specification would be appreciated.

Update: Graham Parks has pointed out in the comments to this post that URIs of the form "feed://http://www.example.com/rss.xml" are not compliant with RFC 2396. This will be folded into the next draft of the spec.