I've known about this for a couple of days but was planning to wait until it was mentioned on the MSN Search blog. However since they've been scooped it looks like the cat is out of the bag. So here goes
Searching for Web (RSS, Atom) Feeds
Need to find an RSS or Atom feed? No problem. Use the feed: operator to do so. This searches for all documents that are RSS or atom feeds
Searching for documents that contain Web (RSS, Atom) Feeds
Need to find a document that contains an RSS or Atom feed? Use the hasfeed: operator.
Folder Level Site Search
You can now use site search (site: operator) to restrict your search to a particular folder hierarchy in the URL up to two levels deep. For example,
http://search.msn.com/results.aspx?q=site%3Awww.microsoft.com%2Fwindows&FORM=QBHP (searching the Windows site on MS.com)
http://search.msn.com/results.aspx?q=site%3Aspaces.msn.com%2Fmembers%2Fmike&FORM=QBRE (searching a blog on MSN Spaces)
Note: There is a known issue around this feature. You cannot include a / after the second directory. This will be fixed in the near future.
So it looks like MSN Search is the first of the big three search engines to provide RSS search and the improvements to the site: operator are also quite cool. They definitely get mad props from me for getting these features out there.