It doesn't work in 2.6 or 3.0 because, as stated above, it was only due
to a bug that it even appeared to work at all in 2.5 (it was always
meant to be disallowed because it puts dodgy data in the import
machinery's internal records if you do it).

It took a new feature (looking for a __main__ module as described in
issue 4195 and as you already noted) to make it actually work properly
for 3.1 (and 2.7 when that is released).