Prior to 1.9, behavior of enumerated hashes was not in the ruby specification and therefore was up to implementation — basically, hash enumeration behavior/pattern was undefined by the language and implementations could really do whatever they want (random? sorted? insertion order? different method every time? anything goes!)
1.9+, hash enumeration is specified by the language to be in the order of insertion, so if you know your platform is 1.9+, you can rely on it.
RubySpec