As a 2004 Republican Presidential Elector, you must sign an affidavit stating that you will vote for the Republican Party Nominee---this year meaning Bush---when you place your Electoral College Vote.
On December 13, 2004, I will wholeheartedly support and place my one Electoral Vote for President George W. Bush and Vice President Dick Cheney.
It's probably not legally binding, and if it is, so what? Peple break the law all the time.