I have had success in getting a refund in these situations. I have always sent a letter to the hotel with a printout of the flight status page showing cancelled and there has never been a problem.

Also, in Las vegas where most if the hotels charge a one night deposit they ALWAYS offer a refund if your flight is cancelled. They have a form in their system that they use and it is an instant re fund. You just need to provide the flight number and the agent will varify that it was cancelled and then give you a refund.