That is great news! Rails 3.1 otherwise has many great features.
However, I'll be honest, I find it completely unbelievable that you don't have a problem debugging in one huge js file. I guess when using coffee script the connection between what you are debugging and what you are editing is already completely severed so maybe at that point it doesn't matter anymore.
We're building client heavy apps, though, and haven't felt a need for it. But now it's there if you need it. Enjoy!