The guys over at AdDuplex are running a competition where you can get your hands on a "not yet announced Windows Phone" - this will be a chosen device released later in the year around Mango. To enter, you are required to use the AdDuplex Ad Exchange service with your WP7 application, but should you already be using the service (which I recommend you do) then you're already entered. Check the website for more information and sweepstakes rules.

Source: AdDuplex