In PhoneDog v. Kravitz, PhoneDog sued their former employee, Noah Kravitz, in regards to his Twitter account. The court ruled on the defendant’s motion to dismiss: http://scholar.google.ca/scholar_case?case=9890904231170613660&q=phonedog+v+kravitz&hl=en&as_sdt=2006
PhoneDog alleged that Kravitz misappropriated his Twitter account that was curated while he was employed at PhoneDog. PhoneDog framed the cause of action as the misuse of a trade secret. They alleged that Kravitz misappropriated the Twitter password, which was a trade secret (as a way to fit the grievance into a legal framework).
PhoneDog wanted control over the account (due to the number of followers) but focussed on the password to get control of the Twitter account.
The proper way to deal with this is through contract. If your employee is managing a Twitter account on behalf of the employer or using a personal account, then there should be a policy on the use of social media accounts.