Am I misusing socket.setdefaulttimeout here?

[Skip Montanaro]

Socket timeouts are available in 2.3.  I'm trying to use them to check URLs
on a couple web servers without hanging indefinitely due to downed servers:

    #!/usr/bin/env python

    import socket
    import urllib2
    import sys

    TIMEOUT = 30.0
    socket.setdefaulttimeout(TIMEOUT)

    f = urllib2.urlopen("http://www.mojam.com/")
    data = f.read()

When I execute the above, it returns immediately (that is, not 30 seconds
later) with this traceback:

    Traceback (most recent call last):
      File "chksrvrs.py", line 11, in ?
        data = f.read()
      File "/Users/skip/local/lib/python2.3/httplib.py", line 1155, in read
        return s + self._file.read()
    IOError: [Errno 35] Resource temporarily unavailable

Do I misunderstand something about socket timeouts or do they not play well
with urllib and friends (I tried both urllib and urllib2).

Thx,

Skip