http://yacktrack.com/ offer a nice interface for searching through multiple blog sites at once including Google blogs, Technorati, Twitter and others. They also plan to add more sites over time.
Choosing the "Chatter" tab and putting in a search brings back results in a simple format ready for sifting through. Run these results through http://www.usernamecheck.com/ and you'll be well on your way to tracking down that person you've been looking for.
Most importantly if you create an account you can save your searches and run them via RSS.
Have I mentioned how much I adore RSS?