Win7 Mobile is not built on previous windows mobile platform but a clean concept. The development tools are new to mobile too. It utilises Silverlight and XNA for application and games development. So it will start with a pretty big developer base.
I was hoping to get a peak at the new Windows Mobile OS from the emulator but all I found was the browser. The UI and other things are there. The ability to compile and tests apps so early is also very nice put none of the rumoured integration with core apps features seem to be available yet. You do on the other hand get to see what the feel of there user journey is like and how the menus and navs will work.
All and all worth a look.
Here is some new Blend 4 + Windows 7 Mobile plugin in for Blend
Here is some nice tutorials.