You cannot copyright a Tweet
Though this may not completely eliminate liability if a publisher or artist rewraps a whole series of your tweets in a different shell, Zeldman makes an interesting point on the limitations of copyright in regards to short phrases (i.e. tweets):
As messages sent via Twitter cannot be longer than 140 characters, they cannot be copyrighted. However original, witty, or profound they may be, nothing more than good manners protects your original expression of authorship. If you wish to let other people quote or use your Tweets, you need not “license” them; indeed, technically, you cannot license them, since they are in the public domain the instant you publish them.