As of yesterday, you can download the new version of the DSL SDK.
We fixed some bugs and we added some really cool features such as better templates for WPF and Winforms based designers, T4 code generation at build time, domain model extensibility (you can even extend the Dsl designer itself), and DslLibraries and much more. For the full list see Jean-Marc's post.
There's a known issue with debugging Dsl designers using F5 - the toolbox items won't appear in the exp hive. In order to work around this (until the RC) you will have to run using CTRL+F5 (without debugging) and after the exphive will showup you will be able to attach the debugger. See this thread of more info.
Have fun