Rollyo has been all over the blogosphere, but I'm going to pile on because it's a damn good idea. As Rael Dornfest puts it, Rollyo supports attenuated search. Using the power of the Yahoo! Search API, you can focus searches on specific sets of sites.
Here's a no brainer, make it easy for people to import blogrolls as searchrolls. Voila! You can do focused searches against sources you already trust to see what they have to say on a topic.
Besides, any site with the luscious Rosario Dawson on board gets double thumbs up from me.