Real ballplayers are prohibited from participating in fantasy baseball - its covered by the prohibition against gambling on their own sport.
However, fantasy football is IMMENSELY popular amongst baseball players. Generally, manager ask them to keep it out of the clubhouse though, because it looks bad.
"One of the teams represents truth, justice, the American way, and underdogs everywhere. The other represents George Steinbrenner!" - U.S. News and World Reports columnist John Leo on the difference between the Mets and the Yankees.