In fact, this very issue has recently made its way to a courthouse. A technology Web site, PhoneDog.com, and one of its former chief editors are duking it out in a legal battle that could impact anyone who tweets from work.
Employee ("Noah Kravitz") is suing PhoneDog over his employment contract. The employer has counter-sued - and here is the interesting part - claiming that Kravitz personal Twitter handle "PhoneDog-Noah" actually belongs to the company and that Kravitz essentially stole it by not turning it over to the company when he left their employment.
According to this article in SlashGear, the company values the Twitter followers at $2.50 a pop - that's $370,000 per year for the roughly 17,000 followers the account currently has. According to the article, Kravitz reports that the company did not ask him to create the account; he did it on his own and posted both work-related and personal tweets to it.
Clearly, the employer's claim on the account is murky at best. But this cautionary tale should give pause to any employee that posts to Twitter regarding issues that are in some way related to his/her work. Perhaps it would be best not to include a specific reference to your employer in you Twitter handle, at the very least.