How can I run a focused Shoulda test/context within Vim?

I'm using rails.vim with MacVim and I can't figure out how to run a single focused Shoulda test or context.

The help docs say :.Rake with a line number should work, but it doesn't (probably because I'm using Shoulda syntax).  Does anyone know of a way to run a focused test/context?

Thanks a bunch guys, I've looked for a solution for a few days, but haven't come up with anything at all.