William Cassidy, of California, is charged with online stalking for posting emotionally distressing Tweets about a Buddhist group and its leader in Maryland. The defense claims that emotionally distressing speech is protected by the First Amendment when it is done on a public platform. All 50 states have statutes that protect cyberbullying, cyberstalking, or cyberharrassment, but Cassidy was charged with the federal crime of Interstate Stalking (18 U.S.C. 2261A(2)).
As always, the appropriate analogy must be sought out to handle this type of conduct. Shanlon Wu likens the Tweets to handwritten notes. Eugene Volokh offered the difference between threatening phone calls and the street corner; one may not call a person directly to make distressing claims, but can address a public audience in a public space. I think this is the correct analogy for this instance where a speaker is located such a great distance from the subject of the comments.
However, if the speaker were in Maryland and the public forum was the only way to reach the victim, the subject would feel the same type of intimate threat as one sent from a cell phone. Twitter allows the victim to be exposed to the distressing comments when he or she is alone, creating the same vulnerability, anxiety, and fear stalking statutes seek to avoid.
Beyond analogies, the issue may come down to statutory interpretation. The relevant section of the statute reads:
(2) with the intent—
(A) to kill, injure, harass, or place under surveillance with intent to kill, injure, harass, or intimidate, or cause substantial emotional distress to a person in another State or tribal jurisdiction or within the special maritime and territorial jurisdiction of the United States; or
(B) to place a person in another State or tribal jurisdiction, or within the special maritime and territorial jurisdiction of the United States, in reasonable fear of the death of, or serious bodily injury to—
(i) that person;
(ii) a member of the immediate family (as defined in section 115  of that person; or
(iii) a spouse or intimate partner of that person;
uses the mail, any interactive computer service, or any facility of interstate or foreign commerce to engage in a course of conduct that causes substantial emotional distress to that person or places that person in reasonable fear of the death of, or serious bodily injury to, any of the persons described in clauses (i) through (iii) of subparagraph (B); 
shall be punished as provided in section 2261 (b) of this title.
Physicality may comfort some, but distance is at the heart of this statute. It seems silly to have a section that prohibits interstate communication if distance is a powerful point for the defense of expression. Second, the statute clearly says any interactive computer service. If the court decides that Twitter is like yelling on a soap box, then the statute may not hold up. The Constitutionality of cyberbullying statutes do not offer much guidance, because the statutes relate to student-conduct and the involvement of educational institutions, a different standard entirely.
Because this type of expression chills other expression (I would have gotten off Twitter and moved to the middle of no where, like Kathy Sierra, if this had happened to me), the Internet may force courts to explicitly and concisely address and weigh the value of certain types of content. High-value vs. low-value speech – let the games begin!