PhoneDog could own PhoneDog_Noah, for example, but not _Noah, let alone NoahKravitz.
This particular case will probably be settled as part of the case about Kravitz' back pay, wehther or not the Twitter suit is related.
Other cases – lots and lots of other cases – are sure to come up in 2012, though. It would save a lot of trouble to create a policy now that is vetted by your lawyers and that gives some benefit to the tweeters who might be leaving, though not so much benefit that you, the employer, lose out on the cost of supporting and promoting the feed or paying the salary for the person who wrote it.
Those things have value and should be given some consideration. They don't override the right of writers to write what they want for whom they want, however, or the right of readers to follow writers they like.
Lists of names might be corporate assets; the short and fragile attention of an audience, however, is an expression of tolerance or enthusiasm for a particular writer, performer or source of content. It's not an asset in even the insubstantial way "good will" is an asset on balance sheets.
"Followers" are not an audience bought and paid for by marketing promotions, direct-mail costs and loss-leader special offers to sign up.
Following a Twitter writer – for now at least – is still a voluntary expression of interest, not accession to the assumption any moment of a consumer's attention is worth money and should be paid for either voluntarily or by force.
The ownership question is more complicated between employer and employee; between writer and audience, it is as clear as the invisibly clean glass on which the snouts of birds and inattentive humans are mashed: an audience owes nothing to a writer but the chance to catch their interest.
A writer owes a lot to readers, beginning with the assurance that the content will be at least as useful, accurate and entertaining over time as on the day they agreed to follow.
Writers also owe their audiences the confidence that he or she won't drop the responsibility or sell the opportunity to write for the audience that has picked out as a favorite one strand in what has become an increasingly tangled, complex, heavy tapestry.
That responsibility will get more critical with the presumed addition of a smothering weight of legal obligations, lawsuits and belief that the attention of a thinking being is so existentially inconsequential that others can buy and sell it without regard for rights of the mind from which it comes.