Sean Lyndersay of Microsoft's Internet Explorer team posed a question to the RSS-Public mailing list: What's the preferred MIME type for RSS feeds?
For legacy reasons it seem necessary to provide support for RSS as both text/xml and the proposed application/rss+xml. I would like to see RSS identified by the latter, but there's not a lot of support for it right now in browsers, etc.
An article describing RSS MIME type woes: www.petefreitag.com
The draft standard for application/rss+xml: www.oasis-open.org