The preliminary documentation for the DLR is included in the IronPython-2.0A1-Doc.zip download at http://www.codeplex.com/IronPython/Release/ProjectReleases.aspx?ReleaseId=438 :

We're leveraging the learning we did on IronPython to extract elements that could be common amongst languages (dynamic type system, hosting APIs, cached method dispatch, symbol tables, ASTs, codegen, etc.), and we currently are working on IronPython 2.0, JScript, VBX, and Ruby to vet the common designs. We'll eventually do (or recruit people to do) PHP, scheme, and maybe one more to believe we really can move language+1 with ease to the DLR.

.... Another juicy quote ...

That runtime needs a great scripting story and UIFx story to compete with the virtuous cycle Flash/EcmaScript enjoy. We also watn to work with a partner on an IDE to eventually make a play for the MS app programmability story so that you can have your choice of dynamic language on a little runtime for scripting, say, Office or VS.