This report almost seems like a bug with urlparse, but it is not. We
have to consider certain cases here.

1) First of all, we cannot equate urlparsing, urlsplit, urljoin with
path normalization provided by posixpath.normalize. The reason is the
url syntax is strictly by RFCs which are different than Operating
system's file and directory naming syntaxes. So, the expectation that
urlparse() should return the same result as posixpath.normalize() is
wrong. What we can at most look is, does urlparse follow the guidelines
mentioned in the RFC1808 to start with and RFC3986 ( Current). 

2) Secondly, in a generic sense, it is better to follow the RFC defined
parsing rules for URLS than implementing browser behavior. Because, the
urlparse needs to parse urls of other schemes also say svn+ssh where a
valid url is svn+ssh://localhost///// and in this case '////' is the the
name of my directory where I have the source code. Quite possible,
right? So, it should not be converted to '/' which will be wrong.

3) And coming down to the more specific issues with the examples
presented in this report,
urlsplit considers the first '//' to follow the netloc
and a single '/' or '///' to be path '/'

>>> urlparse.urlsplit('//')
SplitResult(scheme='', netloc='', path='', query='', fragment='')
>>> urlparse.urlsplit('/')
SplitResult(scheme='', netloc='', path='/', query='', fragment='')
>>> urlparse.urlsplit('///')
SplitResult(scheme='', netloc='', path='/', query='', fragment='')

Having this in mind, follow the examples you have provided:

print urlparse.urljoin('http://www.example.com///', '//')
print urlparse.urljoin('http://www.example.com///', '/')
print urlparse.urljoin('http://www.example.com///', '')

You will find that they are according the parsing and joining rules as
defined in RFC 1808 (http://www.faqs.org/rfcs/rfc1808.html)

The same is with other examples, monk.e.boy. 

If you see that urlparse method has a problem, then please point me to
the section in the RFC1808/RFC3986, where it is not confirming, I shall
work on the patch to fix.

This report, is not a valid bug and can be closed.