I've been planning to whip up some clever hacks to demonstrate a few applications of CEP to twitter feeds, but in the meantime here's someone else's browser-based continuous query tool for twitter, TweetGrid. The interface only always text search queries, and doesn't provide refinement or filtering of any kind, but it does allow one to run multiple queries at once on the same screen (maybe we can call this a "visual join?").

Here's a link that will take you to a page showing three continuously updated streams of tweets for the phrases "complex event processing," "data visualization," and "semantic web."

I like to see things like this in the wild because it demonstrates that there are many persons working on problems for which our technology could be of great assistance, even if they've not necessarily heard of CEP.