In late 2010 TransLink, Vancouver's transit authority, embarked on an experiment to use their customer service resources to tweet alerts about system disruptions. I consider this experiment a success. TransLink gets placed on the defensive almost daily in the media and (especially) on Twitter, and they should be praised for getting a lot of things right about the 140-character platform.
When there were no disruptions, they made note of it. That was good, since being quiet in social media could indicate that there's nobody manning the post. It's also smart that they use the evolved standard of carets and initials to indicate who's doing the tweeting. While it uses 3 characters, it's a nice personal touch for an organizational account.
What TransLink could do better is laying off compressing tweets. I have been a little cynical in my "Expanding TransLink Tweets" series on my Tumblr blog (example), so my real feedback would be this: if you can't say it in 140 characters, but an easy-to-read alert can fit comfortably in 280 characters, write two tweets instead of one. It takes the same amount of time to parse the abbreviations as reading the full words would over two tweets. (And that's assuming you know what their abbreviations mean, since they have used "MVI", which is an acronym they're familiar with as an organization but maybe their customers aren't.)