If you've been looking at the latest commits, effort is being done on adding type annotations to the Coffeescript grammar. Once these are completed, the resulting Javascript should be directly usable by the Compiler in advanced mode (to get the most optimized code with the Closure Compiler you will need type annotations for objects, function parameters and "typecasts"). I'm glad the effort is being focused on type annotations rather than Closure Library idioms (e.g. those under goog.array) at this stage. As someone who has to write lots of boilerplate code to leverage the power of Closure Library components I cannot thank this enough (and don't forget plovr!). Just the Coffeescript class declaration sugar that's working right now is quite powerful in terms of reduced noise in your code.
EDIT: wording.