Ok, I want to play with this some more and blog about it...but to help me to remember I'll post this blurb:
Using DSL Tools, you can create a custom graphical designer that uses your domain-specific diagram notation. You can then create custom text templates that use models created in your designer to generate source code and other files. In this release the validation framework makes it easy to apply constraints to the language, and you can deploy the designer as a standard installer package for use within Visual Studio...
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So what's a domain-specific language, and what is it useful for? I'll have to leave those answers for later.