Windows Phone 7 has been unveiled and MS have announced all details of the development chain at the Mix10 conference.
So this could be a good starting point/collection of online resources as they appear
EDIT: I've removed the speculation tag now that the covers are off
An excerpt from Programming Windows Phone 7 Series by Charlez Petzold just became available online.
Charlie Kindel has started a MSDN blog on WP7 development, along with a video on Channel9.
windows phone twitter account: @windowsphone
For WP7/XNA related information, keep an eye on Shawn Hargreave's blog :-)
Some info leaked and available here: http://www.mobiletechworld.com/2010/02/18/windows-phone-7-series-development-information/
You can get the newly released Windows Phone 7 Series "SDK" from the Windows Phone Developer Home Page
Click the "Download the Developer Tools" on the right hand side to get the bits.
Remember the Windows Phone 7 version of fousquare from the MIX10 demo?
You can download the source code from http://4square.codeplex.com/.
There's a ton of resources out there for Silverlight, and for the most part all of this applies to Silverlight for Windows Phone. My limited experience so far has been quite postive. I took a simple game I wrote for Silverlight and cut and pasted the XAML and code into a new Windows Phone application and pressed F5. It ran without modification, the only thing I couldn't get working was the sound.
If your interested I just posted the source code on my blog.
For XNA there's a lot of information at the XNA creators site.
The forums are pretty good too over here combined they cover most aspects of game and app design and publishing.
Worth a visit.
Jesse Liberty
Silverlight and Windows Phone 7 Programming
Look at this stackoverflow post.
Getting started with windows phone 7
I put together a list of resources to help people get started on Windows Phone 7 that might help as well.
the Microsoft Press WP7 book by Charles Petzold is now online here.