Recently on the GOOS group discussion board there was a good debate about Acceptance Testing. Whilst much of the advice was good it amazed me how many people hadn't looked outside of their own language community for inspiration.
For instance, in the dot net world automated acceptance testing frameworks for web sites where fairly poor. However the Ruby guys nailed this a while ago so why not just use those frameworks? Take a look at WATIR for more information. When it comes to black box testing your application why does it matter what language or framework you use? If anything I'd say it's better to test it using a completely different language.
If you've written your application in C# what's stopping you from using Ruby to test you website? Many people get scared of using something that isn't written in their favorite language. The problem with this is you don't expose yourself to new ideas a new ways of doing things.
Don't limit yourself to one ecosystem.