I admit, for various reasons I thought about writing such an environment in Java, actually, writing a Self-like language in Java which then would be the language to code the world in. But Squeak might be an alternative, at least for a prototype.
Stefan, did you see my post about http://spair.swiki.net/21 ?
I hope that someone might take the initiative and use it to create a complete Self implementation in Squeak.
- Stephen