Actually, I should call it a brand new app from Foursquare. There is a total makeover for this official Foursquare App for Windows Phone 7. BTW, it will NOT update your old Foursquare. You will need to uninstall and reinstall the new one.
This time, they worked closely with Microsoft to bring us Foursquare 2.X. According to Foursquare blog, it is using its own API v2 to develop this app.
I think 4th and Mayor (another awesome Foursquare app) will not be too happy.