Heres a userscript for Greasemonkey I made.
It adds a search box at the top of Google Reader. If you are viewing one feed, it will search it using Google's Blogsearch and show it right on the page(It looks like the expanded view). If you search and you aren't looking at a specific feed, it searches all blogs on Blogsearch(Not just the ones you are subscribed to). I want to make it able to search just those in a specific folder or just those subscribed, but I don't know of a blogsearch engine which has the option to search multiple sites.

Heres the script:
http://userscripts.org/scripts/show/5958