The errors below show up on my XP machine in Python 3.1 as well as a recent build from the py3k branch.  I'm not sure why they *don't* show up on the XP buildbot.

fromfd() ultimately calls dup(), so the underlying problem is likely the same for all the failures.  dup() support was added for Windows in issue1378.

======================================================================
ERROR: testDup (test.test_socket.BasicTCPTest)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "C:\Python31\lib\test\test_socket.py", line 628, in testDup
    msg = sock.recv(1024)
socket.error: [Errno 10038] An operation was attempted on something that is not a socket

======================================================================
ERROR: testFromFd (test.test_socket.BasicTCPTest)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "C:\Python31\lib\test\test_socket.py", line 619, in testFromFd
    msg = sock.recv(1024)
socket.error: [Errno 10038] An operation was attempted on something that is not a socket

======================================================================
ERROR: testDup (test.test_socket.BasicTCPTest2)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "C:\Python31\lib\test\test_socket.py", line 628, in testDup
    msg = sock.recv(1024)
socket.error: [Errno 10045] The attempted operation is not supported for the type of object referenced

======================================================================
ERROR: testFromFd (test.test_socket.BasicTCPTest2)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "C:\Python31\lib\test\test_socket.py", line 619, in testFromFd
    msg = sock.recv(1024)
socket.error: [Errno 10038] An operation was attempted on something that is not a socket

----------------------------------------------------------------------

test_socket.py passes here.
I run: python_d.exe -m test.regrtest -v test_socket

I run it the same way.

I wonder what's different? :-(  I will try to find some time to investigate further.

On Windows, socket.dup is implemented using DuplicateHandle.  The documentation for DuplicateHandle reads:

You should not use DuplicateHandle to duplicate handles to the following objects:

    * I/O completion ports. No error is returned, but the duplicate handle cannot be used.
    * Sockets. No error is returned, but the duplicate handle may not be recognized by Winsock at the target process. Also, using DuplicateHandle interferes with internal reference counting on the underlying object. To duplicate a socket handle, use the WSADuplicateSocket function.

Following the advice of DuplicateHandle's documentation, I reimplemented dup on Windows using WSADuplicateHandle.  Attached is a patch.  With the patch, the tests pass on my Windows machine.

Would someone like to review the patch?

The documentation of WSASocket(), and the code of PyBluez
http://code.google.com/p/pybluez/source/browse/trunk/msbt/_msbt.c#374
both suggest to use the FROM_PROTOCOL_INFO constant.
Otherwise, the patch looks good.

Thanks!  Updated as you suggested and committed as r84450 and r84451.