Ralph E. Winnie, Jr. (Hume & Associates, Washington, D.C.) has published A Separate International Tax Regime for Nonresident Athletes?, 107 Tax Notes 1287 (June 6, 2005), also available on the Tax Analysts web site as Doc 2005-2745, 2005 TNT 108-18:

Athletes and entertainers are among the most transient and mobile individuals in the international community.  The types of services performed on a daily basis and the types of income generated from the provision of those services are quite varied depending on the nature and level of activity  Entertainers and athletes travel the globe to work on movies and television shows, perform in charity events, and participate in and promote sporting events and competitions. Consequently, as an entertainer or an athlete becomes increasingly well-known in the international community, his income may not just be derived in his country of residence but in other countries as well. Therefore, the ties as a resident to a particular country tend to lessen because those individuals may spend a great deal of time abroad earning income in many different countries. This article discusses: (1) the difficulty in taxing nonresident athletes in the United States, Canada, and Mexico; (2) the feasibility of a separate tax regime for nonresident athletes; (3) the difficulties faced by nonresident athletes in paying taxes; (4) the characterization and allocation of the income athletes receive; (5) the possible methods of taxing athletes; and (6) a special income tax and withholding tax regime for nonresident athletes.