You can compare up to three searches, and you can graph trends over 2, 3, or 6 month period.
A tip: the search phrase can be a url, which allows you to compare blog citations.
Here's a comparison of Google, Yahoo, and Microsoft mentions across the blogosphere for the last six months:
That image above is static, but is hyperlinked to the active graph on the blogpulse site. The active graph also lets you click on the trendline to pull back the actual search results.
These graphs track mentions. They don't weight mentions by the blog's importance or readership. Thus, splog terms appear artificially high: Blogpulse shows more mentions of "get rich" than "george bush", which I suspect is due to spam.
BlogPulse: a useful tool to visually monitor buzz across the b'sphere.