I downloaded the Mar2007 version to run it on 10.3.9 (PPC). It starts and I can craeet applications - at least it seems. It does not do anything in reality. Console says:
dyld: ruby Undefined symbols:
ruby undefined reference to _lchown expected to be defined in /usr/lib/libSystem.B.dylib
According to the homepage, it's supposed to run on 10.3.9!