Idea: Use PyCParser to parse and interpret CPython. :)
Status so far:
$ ./cpython.py -V ... Python 2.7.1
PyPy, CPython 2.7 (so it can sort of host itself).
The C data structures itself are compatible with CPython, so in theory, you can even load C extensions and it should work.
Just for fun.
(Maybe, to make it in any way serious: here)
See PyCParser for more.