Hi,
I've seen the code change in /python/trunk/configure.in (rev.  74072): 
it enforces a specific autoconfig version (2.61) in order to build 
python.

Unfortunately OpenSuSE 11.1 (mainstream at time of writing) has an 
updated version of autoconf 2.63 (and I think is true for Mandriva as 
well). 

Wouldn't be better to force the 2.61 being the minimal version for 
autoconf allowing more up-to-date versions to be used?

You can have a look into http://pyvm.sourceforge.net/ where continuous 
build of the python interpreter are stored for further references.

Does the check trigger when building Python, or only when rebuilding the
configure script itself? I think we ship a prebuilt configure script
precisely so that people don't have to care about autoconf versions and
the like.

Yes the rebuild does an autoreconf step.

My understanding is that the reason that this particular version of
autoconf is required is that newer versions introduce mixed line endings
into the generated file, which causes problems with storing it in svn.

The introduction of the version check was prompted by the fact that
several developers used the wrong autoconf version when regenerating the
configure script because they weren't aware of this issue.  In addition
to this, however, having different developers regenerate the file using
different autoconf versions would result in the checked-in configure
script ping-ponging because of differing autoconf output.  Thus it seems
it is better in the general case to require that the configure script be
built with a specified version of autoconf.