James - it depends on the school. I don't think our advisor ever attended a Panhel meeting and she certainly didn't tell us how to vote (she wasn't a sister, so she couldn't). Obviously at large schools like Clemson the advisors are going to be more informed. Some sororities do give the advisor an insane amount of power - some she's just a figurehead. But as browneyedgirl said, when you are in that meeting, you (the collegian) are the one who votes.
Now as far as Greek advisors and administrators manipulating decisions with threats, that's another story.
