If you are one of the unlucky saps stuck in the ASP.net world, this might be a cool project for you. This is an early entry from Microsoft into the GWT/PyPy/etc space. It has a permissive license.
F# is becoming an officially supported Microsoft language which will let you convince your boss that this new fangled workflows (aka monads) powered system is safe to use.
If that's what I think it is, it's more than GWT - it's trying to blend the boundary between client and server. I think it's a bad idea as outlined here:
I agree that RPC is bad, but I don't think it is because it blends the boundary. I think its downfall is that it masks failures.
In this case, code seems to be explicitly labeled as client, server, or both. Sometimes, you really do want both. Consider a calendar which is sent down as HTML from the server for mobile clients, but updated on the fly by javascript in full browsers. Typically, however, your client code is explicitly marked as such and calls into server code with the very proven success/failure ajax callback pattern.
F# is becoming an officially supported Microsoft language which will let you convince your boss that this new fangled workflows (aka monads) powered system is safe to use.