The latest refresh of the Windows Phone 7 Tools is now available! Combined with Visual Studio 2010 (any edition, you don't need Express any more, though if that's what you have it will work) you can create apps in no time flat and run them on the emulator. I couldn't resist:
If you've ever done a WPF or Silverlight app, you can do Hello World in a matter of minutes. I put the picture in there just so I could say I had edited the XAML beyond putting my own name in an attribute. If you'd like to do some serious work, there are all kinds of training resource links on Yochay's blog. Don Burnett has some interesting thoughts on why Blend gives you power and productivity for Windows Phone 7 development and how there's just nothing like it for iPhone development.
I have some fun ideas I want to try that will take a little more than 5 minutes, so I'll report back on that front soon. I'm going to put my phone posts in my Client Development category, since after all, client development includes devices like phones just as much as it includes pure Windows apps.