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.
When you fill out the form enter event code Y5DEQ, enter an event date of 4/6/2011, and good luck!
Note:: Sorry, but this contest is only open to developers in the US.