sys.platform will be win32 regardless of the bitness of the underlying Windows system, as you can see in PC/pyconfig.h (from the Python 2.6 source distribution):
#if defined(MS_WIN64)
/* maintain "win32" sys.platform for backward compatibility of Python code,
the Win64 API should be close enough to the Win32 API to make this
preferable */
# define PLATFORM "win32"
It’s possible to find the original patch that introduced this on the web, which offers a bit more explanation:
The main question is: is Win64 so much more like Win32 than different from it that the common-case general Python programmer should not ever have to make the differentiation in his Python code. Or, at least, enough so that such differentiation by the Python scriptor is rare enough that some other provided mechanism is sufficient (even preferable). Currently the answer is yes. Hopefully MS will not change this answer.