Prompted to finally get back to this by the python-dev discussion about PEP 432 (as separating these out helps keep the moving parts clear when working on the startup sequence)

In response to Eric's question, Modules/main.c is part of the CPython runtime, rather than a C application that embeds the runtime. The "main" in the name refers to the exported Py_Main, rather than a C main function.

The *actual* C level main function now lives in Programs/python.c (previously Modules/python.c) and calls Py_Main after doing a bit of fiddling about with the command line arguments and the floating point settings.