Actually, all you really need to get testem working with Rails is a simple configuration file that precompiles assets prior to running tests. I think a Gist would suffice to get the point across and get people to the answer they want most, but I have a barebones project I was using for experimentation if you want me to polish that for public use.
The Gist is here: https://gist.github.com/4472668
Good proof of concept. But the rails asset pipeline precompile process is dog slow, so I am not sure I want to recommend this setup.