Jeremy Siek is doing some great work on gradual typing, and it may get incorporated into Jython. Here is a presentation, and a video. A similar idea is Gilad Bracha’s Pluggable Types. JavaScript 2 had planned to do gradual typing, though it is unclear if that will survive the collapse of the project.

It seems I usually have negative things to say here, so I wanted to note an example of something positive. Jeremy’s work is Computer Science that is imminently relevant to practicing programmers. I think gradual types are an inevitable feature of future programming languages.