So, the Windows 7 Phone tools are out for everybody to start developing your apps.
Here's the link to the developer portal:
I would like to direct your attention to the following documents, before you start playing with the bits:
Windows Phone UI Design and Interaction Guide
Application Platform Overview for Windows Phone
You should familiarize yourself with those (especially the UI Design and Interaction Guide).
I've been "dogfooding" the bit's for while and will try to start posting a few tips and tricks on the WP7 development which is of course very different from what we've had before. If you have never played with Silverlight or WPF, you could feel as if you are starting from the scratch. It's a completelly different developmenf paradigm. Here's the old post of mine that I made in 2006 when I started working with WPF, but this still holds true for Silverlight:
There's a lot more resources available on programming for WPF and Silverlight now, so if you haven't had a chance to learn it, this is your time :)