My opinion, that's a personal visit. I wouldn't give up any negotiating position, and I certainly wouldn't allude to anything about a risk of paying MSRP (since that's what you'll most likely pay since you stated it was a risk) I would simply state that I've decided that I would prefer a 2011, cancel the deal you've entered into and leave your options open for the 2011. Separate transactions, cancel one and enter into the other.

Do it in person, an e-mail will be forwarded to the sales manager and discussions will ensue, a personal visit makes them act now as opposed to giving them time to think about it (and perhaps come up with an alternate plan that may not be to your liking).

If you feel you need something in writing, do it after a meeting.