Wax is a great tool/framework for WatiN. But for more integration of different automation (web + desktop/mobile, etc.) something like Robot Framework (www.robotframework.org) may be more ideal.
I was thinking perhaps the work gone into Wax could be adapted for a "WatiNLibrary" for Robot Framework, with an library API similar to that of the SeleniumLibrary for Robot Framework (http://code.google.com/p/robotframework-seleniumlibrary/)
so users can switch over to whichever tool works better for them easily.
Robot Framework is based in Python and .NET based test libraries can be implemented with IronPython (currently experimental support) or as remote library that communicates over XML-RPC. I've built a generic library interface using XML-RPC so that one can
create any .NET test library for Robot Framework by just following the framework's specs.
I'm planning to eventually make & release such a "WatiNLibrary" but could definitely use some help or input from others. Contact me if interested, I haven't started a project page for it yet.