I recentely started twittering ( follow me: @promopower ) and it's an interesting challenge to create a message within the 140 character limitation.
It's a great test of copy writing skills, actually. It's a little bit like the old question: Can you fit it on a t-shirt?
I highly recommend writing short messages. I still belive in long copy to make a complicated sale. But to keep in touch with your market and to establish yourself as a thought leader, succinct messages... Tweets... that people can read in a few seconds have become one of the most effective ways to make it happen.