In the Keynote at MIX 10 today the availability of the developer tools was announced. One highlight in the announcement is that the entire Windows Phone developer environment is free, and will be when the product releases.
The Windows Phone 7 Series developer site has gone live so you can grab
You can also get the Windows Phone 7 Series Training Kit for Developers containing four hands-on labs.
Also announced by the team is a preview of “Programming Windows Phone 7 Series” authored by Charles Petzold. You can grab a content preview with 6 chapters of content from http://charlespetzold.com/phone/index.html
Finally, Charlie Kindel has posted about all the announcements today at MIX10.