I think that Buffalo is actually the most credible replacement for Rails that I've seen:
The lead developer is a ex-Rails developer with a very keen sense of what made Rails successful. One of the reasons that it's still pre-1.0 is that it won't ship until the plugin architecture is good, which speaks to the point of the article.
You don't need advanced syntax in the server code to write Rails applications, so Go may be a good fit for a Rails replacement.