There has never been a better time to be a Windows Phone developer. If you are a UK student you can take part in a Student Competition where you can win prizes just by submitting applications to the Marketplace. Since you can also sell these applications, and it costs you nothing to join the Marketplace if you register via DreamSpark this seems like a pretty good deal. Find out more here.
If you are old like me and therefore no longer a student you can instead sign up for the Think.Dev rewards programme and get prizes and rewards for submitting apps. This is open to all Windows Phone developers in the UK and you get stuff for just turning up and submitting apps. I might even win a prize for Cheese Lander (although this is probably unlikely). Sign up here.