Martin, at the time I read the python-list thread, I didn't pay any 
attention to the posts' authors.  Only now did I realize you were one 
of the posters.  Oops.

I already know the basic ideas about creating MSIs with Wise from a 
past job.  So now I researched and thought about this particular 
problem for several hours.  I've learned that Windows Installer is able 
to add ...\python and ...\python\scripts to the PATH during 
installation.  It cannot[1] remove them at uninstallation.  [ WiX, and 
its superior competitor MAKEMSI, each provide slightly higher-level 
abstractions[2][3] on top of what Windows Installer provides[4] to make 
this slightly easier, but not by that much.  But I will assume you 
don't plan to spend days or weeks of your spare time on migrating away 
from msilib. :-) ]

Here are the rough notes I've made up so far on how to do this:

-  make a new method add_environment.  In it:
    -  call start_component to create components "modify_path_per_user" 
and "modify_path_per_machine"
    -  call add_data to create an Environment table.  It should have 
two rows[5]:
        -  Name:"=PATH" Value:"[TARGETDIR];[TARGETDIR]\Scripts;[~]" 
Component:"modify_path_per_user"
        -  Name:"=*PATH" Value:"[TARGETDIR];[TARGETDIR]\Scripts;[~]" 
Component:"modify_path_per_machine"

Another difficult part is the UI.  Then there's the issue of switching 
which of the two components are installed based on whether it's per-
user or per-machine and also based on whether the user specifies via 
the UI that they want their PATH changed.  I have to think more about 
that, and I'm already tired of researching.

Remember that Windows Installer cannot undo its PATH changes at 
uninstall time.  So, before I consider proceeding further, let me 
verify a few things with you.

1.  [TARGETDIR] will stay on the path.  I think that is fine, since the 
python.exe will be gone, so will never be executed.  Do you agree?

2.  [TARGETDIR]\scripts will also stay on the path.  And it may still 
contain scripts installed by the user or by third-party installers like 
the SCons installer.  I don't know enough about how Python works to 
know if that's a problem.  Is it a problem?

P.S.  Would you prefer to discuss this by something more synchronous 
like telephone (I will pay the tolls) or instant messaging?

P.P.S.  Now that I have realized how complicated Python installation 
actually is, and how hard it is to design the tables of and write 
raw .MSI files, I have a new appreciation for the work you've done on 
making a Python MSI installer.  Thank you very much for having done 
so.  Also, now that I have started researching how much work is 
necessary to get this done, I realize why you don't want to code it 
yourself.  :-)

I don't know if I will end up actually managing to come up with a patch.

^ [1].  I have inferred this fact based on 
http://www.isg.ee.ethz.ch/tools/realmen/det/msi.en.html -- scroll down 
to the "Setting the PATH" section
^ [2].  http://blogs.technet.com/alexshev/archive/2008/03/28/from-msi-
to-wix-part-13-installable-items-environment-variable.aspx
^ [3].  http://makemsi-manual.dennisbareis.com/path.htm
^ [4].  http://msdn.microsoft.com/en-us/library/aa368369(printer).aspx
^ [5].  Search inside the page http://msdn.microsoft.com/en-
us/library/aa368369(printer).aspx for "If the package can be installed 
per-user or per-machine" to see why you need two rows.