Thanks for checking out our video! There's a number of reasons why we chose to develop for the WP7. From a commercial perspective it's an unsaturated market. From a developer perspective we already have experience with XNA. From a curious perspective, it's a new piece of technology and it definitely looks like the gamers answer for smartphones.
We're definitely impressed by the WP7 and we applaud it moving away from imitating the iPhone like other devices :)