Kathy Sierra, in a well-informed and insightful post, describes the likely psychological impact of Twitter.
One of Skinner's most important discoveries is that behavior reinforced intermittently (as opposed to consistently) is the most difficult to extinguish. In other words, intermittent rewards beat predictable rewards. It's the basis of most animal training, but applies to humans as well... which is why slot machines are so appealing, and one needn't be addicted to feel it.
Sources: Is Twitter TOO good? - Kathy Sierra
It is frightening to think that the steady stream of pings into our consciousness may go over the safe threshold. (The Terminal Man?)
I switched off Twitter after a couple of days of experimentation. It now maintains a quiescent existence in my list of contacts. But there are other contacts and they could collectively produce the same effects as Twitter.
Time for coffee with a friend!