Only users with a coupon code know this URL. The URL parameter won't show up in a Google search, because the canonical is in place. The coupon code works only for that session and will automatically show up on the checkout page.
Works very well for us.
More info: http://googlewebmastercentral.blogspot.com/2009/02/specify-y...
The big concern in implementing techniques like this is that the number of (a) abandoned carts rises due to people who have a coupon yet can't figure out how to use it (or didn't read the instructions in your email), and then (b) the number of support requests will rise. If "us" is a company with 1000 customers, I'm not sure that you've tested the scalability of it with promotional emails sent to, say, 50,000 people on a Tuesday morning. Support for shopping cart-related frustrations is one of those "You'd better provide a response ASAP or you will probably lose the sale" types of problems.