> I don't understand your objection.
> It sounds like you're objecting, but
> then suggesting an implementation?

This sounds like a kludge addressing a transient
problem.  It also feels like a solution catering
to developers at the expense of user confusion:

1. Why is there PYTHON3PATH, but not PYTHON2PATH?
2. Is PYTHONPATH effectively PYTHON2PATH or PYTHON1PATH?
3. Will PYTHON3PATH be supported forever or deprecated once 2.8 and 3.2 
are close enough to support a large common code base?

There are many ways to address multi-python site issues. PYTHONPATH is 
too indiscriminate for some uses.  When more fine-grained 
differentiation between 3.x and 2.x files is needed, a better approach 
would be a custom loader in metapath that can look for version specific 
file extensions.

I would suggest to wait until there are enough reports from the field on 
different solutions in actual use before selecting one to standardize 
on.

[I have to mention that perl 5 uses PERL5PATH, but I am not sure whether 
it is an argument for or against PYTHON3PATH.  Can anyone report on pros 
and cons of PERL5PATH from actual experience?]