I am just learning about how effective Twitter might be in the classroom. A friend of mine at St. Theresa of Lisieux CHS got his students to find a real-life example of a science concept they were studying, take a photo, and "tweet" it! Isn't that awesome? The students won't ever forget it!
From a PD perspective, I have been following a fellow named Steven W. Anderston@web20classroom and have found his links to be very useful. His blog has tons of really great info. for educators.
Link to Steven W. Anderson's blog