os.path.join('c:', r'\\server\share') returns
c:\\server\share rather than \\server\share. 
Interestingly os.path.join('c:a', r'\\server\share')
returns r'\\server\share'.  IMHO, r'\\server\share'
should be returned in all cases.

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This is a difficult issue, since as far as I recall Windows
allows two or more backslashes in a row in the middle of
normal paths.

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I'd recommend not to change the behaviour, but add a note in
the docs that UNCs are not recognized as such in os.path.join.

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If the current behavior is kept, then os.path.join can't be
relied upon to return the 2nd (or 3rd+) argument if that
argument is an absolute path.  This is one of the things I
rely on os.path.join to do and would need to write my own
version.  Also, the behavior is quite different if the first
agument contains more than simply the drive letter.

I know this is a difficult issue and that is why I would
like to see it fixed in the standard library.  My question
is whether there is a rationale for the current behavior.

Seems to be fixed in python3.1. See #5799. This is not backported for
2.x yet. (Or it won't happen)

Can the fix be backported to 2.7 or can this be closed?

I say close it since it is a change in behavior and shouldn't go in a point release.  I'm happy to see it fixed in 3.1.

It seems that joining UNC path to a directory root fails:

Python 2.7.3 (default, Sep 26 2012, 21:51:14) 
[GCC 4.7.2] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> from ntpath import join
>>> join('c:\\', '\\\\server\\mount\\dir')
'c:\\\\server\\mount\\dir'

When the first argument contains also something else everything works fine:

>>> join('c:\\dir', '\\\\server\\mount\\dir')
'\\\\server\\mount\\dir'

I would say this is a bug. Obviously fixing this changes behavior, but I have hard time believing that anyone relies on the current buggy output.

On the other hand, I was today reported a bug in my software that was caused by this. Funny thing was that the bug was in code that was added as a workaround for issue #3426 (i.e. use `join(os.getcwdu(), path)` instead of `abspath(path)` when path is Unicode). Luckily adding another workaround for this issue is pretty easy.