more on rss feeds

Maybe I've said this before but I really want an application that looks like a feed reader and pulls RSS content from weblogs and news sites but which is topic based rather than URI based. I want to be able to subscribe to term or phrase and get specific RSS posts which match against that term or phrase. Right now I use web-based search tools like Feedster to achieve this but I long for the rich client environment of an email or feed/news reader. Anyone know of anything like this?

I firmly believe that I'm not alone in wanting to harness the immediacy and variety that is available from weblogs and news sites via RSS but not the least bit interested in the particular blogs or news sites themselves. Give me all of the news and postings that match "mars + space" or "mozilla" or "gulf + war". Surely there's a market for a client that pulls based on topic rather than URI, maybe even a larger one than exists for the current crop of feed readers.

I suppose it would require a server somewhere with a database that indexed feeds or maybe the user would subscribe to lots of feeds and then the client could search locally. However it's done, I think that a client that did this would be wildly popular. Maybe a Mozilla Firebird or Thunderbird extension that worked with the Feedster database would be just the ticket :-)