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Problem source confirmed: reverting the PEP 302 breakage of
r46372 (need-for-speed import cache hack) fixes the problem.

setuptools 0.7a1 doesn't show any problems because it uses
2.5's "pkgutil" module if possible, and r46372 patches
pkgutil in a way that masks the problem, at least as far as
setuptools is concerned, for this one specific feature. 
(Other code in setuptools is affected in both 0.6 *and* 0.7,
and will require further workaround code to be written.)