If you use Twisted (or any other network software which uses non-blocking sockets) and you handle large numbers of concurrent connections, you probably want to avoid Python 2.4.3:

exarkun@kunai:~$ python
fPython 2.4.3 (#2, Apr 27 2006, 14:43:58) 
[GCC 4.0.3 (Ubuntu 4.0.3-1ubuntu5)] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> files = []
>>> for i in range(1024):
...     files.append(file('/dev/null'))
... 
>>> import socket
>>> s = socket.socket()
>>> s.connect(('www.google.com', 80))
>>> s.send('GET / HTTP/1.1\r\n')
Traceback (most recent call last):
  File "", line 1, in ?
socket.error: unable to select on socket
>>> 

Hopefully this will be fixed in Python 2.4.4 and Python 2.5.0. Monitor the progress of this bug on the sourceforge bug tracker.