He put the clause in because he wanted an out if the new owner turns out to be very hands on(like Jerry Jones). I think as long as Ross stays out of his way(which he vowed to do), Parcells is going to stay.
I know Parcells is not always a man of his word, but I do believe him when he said it isn't about money any more and that he has too much as it is.
Lets look at it like this. Parcells loves football and wants to be able to have control of a football team without having to do the daily work of a GM or head coach. He has all that power in Miami, and at this stage of the game, he won't find another team with that offer. He could opt out, take the money, but is going to sit out the 2009 season. Only the Chiefs head coaching job(not official yet) and Raiders coaching job are left. The Raiders are a mess and Parcells won't have the same power he has here. The Chiefs already have a new GM, so again he would lose a lot of power.
Despite all the reports and even though Parcells can never be 100% trusted, I think the chances of him actually leaving are less than slim. He has everything he ever wanted in Miami and he has HIS guys running the team. I highly doubt he would just take his money and go sit on the couch for the next year.