I've started to review Silverlight4 + RIA services and got some minor problem following some of the videos/website.
Specifically I could not find the DomainDataSource.
While watching this great video by Tim Heuer I got stuck when he starts talking about the DomainDataSource.
For some reason DomainDataSource was not in the toolbox. Furthermore, I could not even find it neither on Add new reference dialog nor in the GAC.
If found that assembly at:
C:\Program Files (x86)\Microsoft SDKs\RIA Services\v1.0\Libraries\Silverlight
So this post might save you some searching minutes..