To make it easier to create merged resource dictionaries in Silverlight 3 beta, I put together a little Visual Studio item template so you can do "Add | New item... | Silverlight Resource Dictionary".

For C# users, take the attachment & copy it into "C:\Users\<your name>\Documents\Visual Studio 2008\Templates\ItemTemplates\Visual C#" .  Look for the template at the bottom of the dialog in the "My Templates" section.

Enjoy!