Google launched their blog search a few days ago. As with everything Google launches, it is set for a huge success.
Six Apart published a very nice description
First, the new Blog Search works. All the basic functions you’d expect from Google search results are present, including ranking results by date or by relevance. (Interestingly, the default is by relevance, like other Google searches, instead of by date, which is the default for most blog displays.) But more importantly, the advanced search offers powerful functionality such as searching by date ranges and limiting to individual blog authors, in addition to features like searching for words in a blog post title or by language, which have been deployed in the past on other services.
The new features in Google Blog Search are useful because of the (perhaps subtle) distinction in how it works, compared to the traditional searches powered by Google’s googlebot indexer. Google Blog Search works by crawling XML feeds, rather than simply crawling the HTML output of a blog. Because feeds are, at least ideally, better structured than the published HTML of most blogs, it’s possible to extract information like authorship of a post in a fairly consistent way.