It’s important to note that from what I have read on Search Engine Watch and others Google is specifically restricting its search to feeds, and not using the HTML of the blog. Why? Googlebot is designed to swallow a page whole and not break the page up into individual entries or items. Feeds come prepackaged as individual items or entries allowing for easy digestion by parsers and indexers. Google would need to overhaul its indexer or design a new and separate indexer specific to blog posts if it would like to include more post content than it is currently pulling down from a page’s link alternate declared feed (this is based on a conversation I had with Google engineers in February about the indexer, I won’t blog the details, and things may have changed). Technorati indexes a blog’s HTML assisted by the declared RSS and Atom feed, so I am admittedly a bit biased.
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.
More on the actual Google products as they become available.