Ira P. Robbins (American), Digitus Impudicus: The Middle Finger and the Law, 41 UC-Davis L. Rev. ___ (2008).  Here is the abstract:

The middle finger is one of the most commonly used insulting gestures in the United States. The finger, which is used to convey a wide range of emotions, is visible on streets and highways, in schools, shopping malls, and sporting events, in courts and execution chambers, in advertisements and on magazine covers, and even on the hallowed floor of the United States Senate. Despite its ubiquity, however, as a number of recent cases demonstrate, those who use the middle finger in public run the risk of being stopped, arrested, prosecuted, fined, and even incarcerated under disorderly conduct or breach of peace statutes and ordinances.

This Article argues that, although most convictions are ultimately overturned on appeal, the pursuit of criminal sanctions for use of the middle finger infringes on First Amendment rights, violates fundamental principles of criminal justice, wastes valuable judicial resources, and defies good sense. Indeed, the Supreme Court has consistently held that speech may not be prohibited simply because some may find it offensive. Criminal law generally aims to protect persons, property, or the state from serious harm, but use of the middle finger simply does not raise these concerns.