Have you published an app to the Windows Phone marketplace recently? Are you about to? Have you got 4 apps already in the marketplace?
There is a contest going on right now until June 30th to win a free Windows Phone 7. All you have to do is get an app into the marketplace, and fill out a simple form. Each time you release a new app you can submit another form and increase your chances of winning. I don't have exact numbers, but my understanding is that the chances are pretty good. If you've already got apps in the marketplace then use those for the contest as well.