I think you will find that the contractual obligation of the airline is that they will get the passenger from point A to point B, and not that they will depart at any set time, or carry them on any specific flight number, on a specific aircraft type, in a specific seat or even in a specific class.

I don't belive that contractual obligation is any different if the cause of the change/delay is due to weather, maintenance, aircraft change, industrial action, terrorism, act of God, security breech, fire in the terminal, or overselling.

Perhaps one of our regular legal types can clarify - off the record of course.