Ruby is slower than PHP. Ruby can be 20x slower than Java in fact. There'd be absolutely no point rewriting it. I'm really interested to see where things like
http://www.webtoolkit.eu/wt go, though. C++ is pretty fast.
The point of RoR is that it's fast to
develop in it, that's about its only selling point - if you have an already developed system that works just fine it would be completely redundant to rewrite it.