Over the last year I have been working on .NET RIA Services, a preview of which just went public at MIX.
Brad Abrams does a great job on his blog of answering the obvious question you might have - What is .NET RIA Services?. Also be sure to check out his session at MIX on the same topic.
Nikhil Kothari presented a great session on .NET RIA Services at MIX as well.
A good resource for getting started with RIA Services is this wakthrough that guides you through building your first Silverlight RIA Services Application. Once you are ready to dig deeper into individual feature areas, this doc should help with that. If like me you learn by stepping through working code, you will find the sample applications here helpful.
As with any preview release feedback is welcome and greatly appreciated.