I think feed search is just another type of search restricted to a group of MIME types, as previously stated, and as the types of content made available via a feed continue to grow I think a feed-specific search tool will become much more than just blog search.
Niall also talks about how Technorati indexes the entire blog page HTML, not just feed. This is actually less valuable for me -- all I ever get that way are blogroll hits and other irrelevant matches.
In exploring microformats, I actually was checking out Google's support for filetype restrictions. For instance, here is a search for the phrase "test", restricted to vCards (.vcf is the extension). Very interesting...
So, the space continues to evolve. In my TV spot, I demo'd how searches for London Bombing at the time showed results from the 1940's blitz in Google, and realtime results in blog search engines. Google had to respond at some point. This current release is decidedly uninteresting -- the results aren't that great. I do like the hinted-at capabilities of the advanced blog search page, but PubSub's boolean operators are already as powerful or more so.
Aside: Hey, are PubSub SiteStats new? here are the ones for this site.
So, Google, welcome to the community. I'm hoping you'll grow a Zawodny soon...you're starting to feel a bit big and corporate (translation: it doesn't feel like you're engaging your community)