I just noticed this new C9 series yesterday and added it to my list of WP7 resources but think it’s worth a separate callout here. If you’re completely new to development (unlikely if you’re reading this) or you know someone who is but fancies getting started by writing an application for Windows Phone 7, then Windows Phone 7 Development for Absolute Beginners should fit the bill perfectly.
Clint and Bob take you through 64 separate topics walking you through how to get started in development using Windows Phone 7 as as the target platform. Some example topics include:
As you can see, it ranges from some very fundamental concepts to explaining capabilities specific to the Windows Phone 7 platform. It might even be worth dipping into if you’re familiar with .NET and just want to understand one or two specific areas.