An updated version of the UI Design and Interaction Guide for Windows Phone was posted on the Windows Phone 7 forums today. There is also a big collection of Photoshop files to help when designing your apps which can be downloaded here as well.
Take a look through the new guide and feel free to post anything you find interesting either here or at the official forums. The guide has gone from 69 pages to 101 pages, so there should be lots of expanded content to look through.
Below are some sample shots from the updated guide. You can find the 10 items that can be shown in the Status Bar, the push notifications whose colors conform to the user's accent color, and the updated application bar which now shows button labels - hopefully you've already noticed most of this playing around with the most recent release of the developer tools.
You can also check out all the available accent colors in the updated guide. My guess is that these are the final theme choices, but I wouldn't be surprised if different phone carriers add in their own company colors and perhaps set them as the default.