I've been busy implementing new Python features and fixing bugs this week. The type checker now supports a pretty good Python subset, even though some important functionality is still missing. A week or so ago almost each new piece of real Python code that I adapted to static typing unearthed a bug or two, but now things are significantly better (though there will be bugs, trust me).
More importantly, I believe that mypy is starting to be good enough for experimentation by outside developers. I've decided that the source release is tomorrow (Friday), barring something unforeseen such as a December blizzard that leaves me out of electricity. ;-)
Remember that the current implementation translates mypy code to Python, so there is no performance boost. However, the current implementation should give you a good idea of what it is like to program with mypy and static typing (though obviously this is just a prototype and the experience will get better!). I'm starting to shift development efforts towards the native code compiler, which is the really interesting bit.