I recently subscribed to the Windows Phone Marketplace. Thats one of the requirements to fulfill if you need a developer phone :) Not that I have any hope on receiving one.
Anyways - I am first in line for a release phone... I love the GUI approach these devices are taking.
Look at : http://www.windowsphone7.com/ for some more details if you are interested in a very good alternative mobile plattform.
Registering on the market seems to be rather difficult. You need to jump through some holes to get the certification process done. That was way easier on Googles market but there I dont have a payed app out :)