In 2004 I had already been using Kawa Scheme on the JVM in my day job for 4 years. That's also the year I began OurDoings using the same technology.
There's also SISC, which purports to be a full implementation of R5RS Scheme for the JVM.
Are you also familiar with Clojure? How would you compare the two?
I'm not familiar with Clojure, but as I understand it's a new language. Kawa is R5RS Scheme except it can't capture continuations. Kawa has the advantage of being a fast compiler. SISC is an interpreter, but it is more completely standards-compliant Scheme.
These days (article is from 2004) we have the luxury of being able to use expressive languages within a large library ecosystem by using implementations of them in the giant platforms that have been built by Sun and Microsoft. Personally, I'm using Clojure, a Lisp dialect on the JVM, for all my explorative programming. Yegge himself seems to be working with Rhino (JavaScript on the JVM) these days.