Steve sums up everything on his blog, but Ill quote him just for those who dont want to click through

Type in a phrase into Google.com, add the word “blog” at the end of your query and not only will you get web results but more up-to-date gems from Google Blog Search too.

His source for this is the Google system blog.

This is yet another shortcut for your searches. Blogs have taken over the world; even my local newspaper has them. The ability to now simply search in a logical way straight from Google is great. Again, if you are not using Firefox, get it, the Google search is built right in!