I have used $r with version 3.11 GA and it works fine for me. Can't seem to find API reference for any special variables like $r, $0, $args etc though.
Jaikiran, I was refering to the API documentation similar to ones like http://java.sun.com/j2se/1.3/docs/api/index.html . API pages usually have a 'since' attribute for class functions and fields which will specify the earliest version of the API with that particular function/field.
I was looking for a reference to $r or any special Javassist variables in Javassist's API page in http://www.csg.is.titech.ac.jp/~chiba/javassist/html/ but could not find it.