This isn't even an issue. Bill Clinton ran for governor of Arkansas in 1990 and promised to serve his full term, but that campaign promise became irrelevant the day after the election.
The bigger issue is that Hillary must be defeated in 2006. She'll have a hard time winning a nation-wide election in 2008 if she couldn't even win in one of the most liberal states in the U.S. two years earlier. There is some speculation that she wouldn't even run again for the Senate in 2006, opting instead to run for governor of New York. A governor generally makes a far more formidable presidential candidate than a Senator, anyway.