"...I'm sure there's a dozen more languages that allow you to redefine at runtime"
Indeed. There are all the Lisp dialects to start with.
And it's no surprise that hence SmallTalk had this feature too: Alan Kay (+), one of the key creator of SmallTalk, acknowledged the heavy influence of Lisp on SmallTalk:
http://www.franz.com/services/conferences_seminars/lisp_50th...
(+) Famous quote attributed to Alan Kay: "I coined the term "object oriented". I can tell you I didn't have C++ in mind."