Gallio and ReSharper 5.0
It turns out that the 3.1 builds listed on the downloads page aren’t new enough. They only support ReSharper 4.5. For ReSharper 5.x support, you’ll need to grab a v3.2 build from the development builds page. At the time of writing, Gallio v3.2 build 437 was the latest available.
That said, I’ve still not managed to completely get my head around NBehave. More on that later.