Eldar Murtazin from Mobile-review had a chance to play with a working build of Windows Mobile 7. According to his comments on Tweeter, the OS has been built from scratch and none of WM6.x apps work on it. Bummer. He also mentioned that there's "a lot of horizontal movements, a lot of additional info by clicks... for WM users it will be a great step ahead. For market it's a copycat of Android 3.1/3.2 or iPhone (he probably meant Android 2.0/2.1)".
I guess Windows Marketplace for Mobile will need a department for Windows Mobile 7 compatible downloads. We'll know more about the OS at the upcoming MWC show next month.