It is a common thing in programming languages, and a sign of maturity to implement a language in the language itself. This however is not common in scripting languages. Most scripting languages remain implemented in the host languages until the end. (This is mostly because scripting languages are interpreted, and the idea of having an interpreted interpreter doesn’t sound so good)
This is not the case again for Python! This started as an idea Christian Tismer had. Python is such a flexible and mature language that it would be really cool to be implemented in Python. This was the beginning of the PyPy Project. To go against the trend and put aside the fears of the speed issues takes a lot of guts, and I can asure you Christian has it. He really proved this before with his dedication in the creation of Stackless Python.
So as the website says the PyPy project is about creating a flexible but also fast Python implementation written in Python. This version of Python will have the Stackless concepts embedded, and since is will be implemented in Python the implementation will be more clear since no messing with the C stack will be necessary.
Anyway I’m really looking closely to that project, will it make it up to the rumors that the secreat goal is being faster-than-C?