I've begun posting to Twitter consistently (you can follow our PointAbout account here, or my personal DROdio account here) and I just had a fellow Realtor email me saying, "I noticed you're now following me on Twitter. As I just signed up for it, I'm feeling a bit like the proverbial dog who caught the car. How best to use it is my question to you. Pleased with any feedback you can provide."
So in the spirit of answering emails with blog postings, I've found a posting created by Creative Commons called "Twitter in Plain English" that really does a wonderful job of explaining why you should care about Twitter, and how to use it.