Just wondering, why would you call WatiN a unit testing tool? Last time I checked, it ran integration tests.
The best way would be to move all code that doesn’t depend on HttpContext to a separate assembly and run unit tests as usual. The rest can be tested with Ivonna. She doesn’t test the client behavior, that’s where WatiN can be helpful; however, if you want to test your pages or controls in isolation, she’s your only choice.