First of all, if you got a good price, keep in mind that even if it doesn't work out for you for 2010 you'll have many wonderful years to use it there.
You have several possibilities to make it work though:
You can ask the owner to see if any alternate dates are available. It only costs $29 to change a reservation. With both of my units, the previous owners had reserved good weeks- but they weren't ideal for me. Both times they made changes for other peak weeks, in one case about 3 months before and in the other about 9 months before. Since MGO didn't overextend the season, you may very well luck out, especially if you have a wide array of dates that would work.
Otherwise, you still have plenty of time to make a deposit in II and try for an exchange either there or somewhere else for next year. Once you become an owner I'd try Marriott again for a change- you never know when a week gets canceled.
Good luck- hope you got a great deal!