I take chapter "19.12 base64—RFC 3548: Base16, Base32, Base64 Data
Encodings" (Python 2.6) as an example. At the bottom of the chapter, you
find references to other resouces, in this case:

See Also:
Module binascii Support module containing ASCII-to-binary and
binary-to-ASCII conversions.
RFC 1521 - MIME (Multipurpose Internet Mail Extensions) Part One:
Mechanisms for Specifying and Describing the Format of Section 5.2,
“Base64 Content-Transfer-Encoding,” provides the definition of the
base64 encoding.

Interestingly enough RFC 1521 points to a web page (the link has been
removed in the current HTML documentation).

However "Module binascii" does not link to binascii in the library.pdf,
whereas the HTML documentation nicely links to
http://docs.python.org/library/binascii.html.

The file base64.rst IMO indicates that a link should be produced:
.. seealso::

   Module :mod:`binascii`
      Support module containing ASCII-to-binary and binary-to-ASCII
conversions.

   :rfc:`1521` - MIME (Multipurpose Internet Mail Extensions) Part One:
Mechanisms for Specifying and Describing the Format of Internet Message
Bodies
      Section 5.2, "Base64 Content-Transfer-Encoding," provides the
definition of the
      base64 encoding.

Should be fixed in Sphinx rev 735:a4019921bdf4.

Hi, I just checked out sphinx version 67171. It still seems not to
procduce a link to the module. How does Sphinx rev 735:a4019921bdf4
translate into a Python revision?

This is what I did:
 $ make update
svn update tools/sphinx
At revision 67171.
svn update tools/docutils
At revision 67171.
svn update tools/jinja
At revision 67171.
svn update tools/pygments
At revision 67171.
...

make latex; cd build/latex; make

It doesn't directly translate. I'm going to switch the Python docs'
build setup to checkout from Hg soon.