Simon,
You said "Java is fast becoming de-facto standard". I hear Java folks saying that all the time (in fact, so much that "de facto standard" is becoming more common a phrase than "sliced bread"). But Java folks never say what Java is becoming a standard for.
I like the idea of Jacl and ns_java. It gives you the speed and ease of use of Tcl for most things, and when it's necessary and of good measure, it lets you use Java for things where it's better suited.
Having used Jython, I can say it's very nice to have that integration.
