update: by Ruby I mean the standard Matz codebase.
http://www.freshports.org/lang/ruby18/
Maybe this is a lesson not to blindly use ruby without going through a trusted distributor. Not unlike linux?
Moreover, from reading the advisories and comments, the only thing I can tell for certain is that you can kill a Ruby process with something like "Array.new[0x7fffffff]=0". The only way that's going to take down a Rails app is if you're putting unchecked user input directly into array indices -- but then, if you're doing that, you're probably in trouble anyway.
Or if you're running a setup like Heroku, and you let users run a Ruby console. (Maybe. I don't actually know how Heroku works.) To the nearest percent, that describes 0% of Rails apps.
Can anybody describe how this would actually be used to attack a typical website running Rails?
An attacker could study the code paths of popular ruby based open-source projects to craft special attack inputs that exploit the weaknesses in Array and/or String.
As a hacker with the freedom to be creative with how I work on web applications, I can choose Ruby because I'm not an enterprise architecture astronaut working on Serious Applications.
The ruby thing was annoying but hasn't stopped me writing and deploying web apps in Rails. Hopefully it won't happen again.
Instead of living in perpetual fear of being hacked, I'm out there doing what is more important to me: creating great web applications that users want to use, in a language I love using.