If you’ve been using the beta tools to develop for Windows Phone 7, you’ve had to do without for a little while – but no longer.  The RTM tools are available for download!  There are a ton of great things included in the package and they have been and will remain completely without cost.  So go get ‘em and happy developing!  Here are some more great resources to get you going ...

Windows Phone 7 Training Kit for Developers - RTM Refresh
Silverlight Windows Phone 7 Toolkit
Learn Expression Blend
MSDN Docs
Microsoft Advertising SDK for Windows Phone 7
Great "Tips and Tricks" post by Tim Heuer
Windows Phone 7 Design Templates
Tons of links on WP7

Some good info on using oAuth in a WP7 app. specifically with Twitter:

Building a ‘real’ Windows Phone 7 Twitter App Part 1
Building a ‘real’ Windows Phone 7 Twitter App Part 2
Building a ‘real’ Windows Phone 7 Twitter App Part 3