One day I walked past a store in the mall which had a big flat-screen TV displaying a feed of customer feedback from the web. At the time I wondered how they did that and thought it would be a cool thing to incorporate into PD session or conference. Yes, you could simply use Twitter or TweetDeck, but Wiffiti provides a more of a polished display for Tweets in addition to incorporating non-Twitter based mentions. Now that I think back on the display that I saw, it may have been powered by Wiffiti.
Here's a short video clip of Wiffiti in use at a David Archuletta concert in June of 2009 courtesy of YouTuber Stepploca.
In addition to scanning Twitter, Wiffiti also offers the option of including SMS (text) messages sent to a custom number for incorporation onto the Wiffiti screen. A possible classroom application would be allowing students to text questions for display on the screen in real-time (A potentially dangerous but fun idea!).
Here is an example "Wiffiti screen" of the Twitter Hashtag #gct (Google Certified Teachers).