Pat Sajak (yes, the Wheel of Fortune guy) has some reasonable thoughts about celebrities expressing their political views.
I especially liked this bit:
A celebrity should try to consider the appropriateness of a venue before opening his or her yap about political and social issues. Just because an arena is full of screaming kids who have come to hear your latest songs doesn’t mean you have the right to abuse this captive audience with speeches, tirades or political proselytizing. When you go up to a bank teller for a transaction, you don’t want to hear a lot about politics or the environment before your check is cashed.