#no mention about formatdate at all:
% pydoc rfc822 | grep formatdate | wc -l
0
# but rfc822.formatdate docstring exists:
% pydoc rfc822.formatdate | wc -l
12
# also same behavior :
% pydoc codecs | getwriter | wc -l
0

Reproduced at:
Python 2.5.1 (r251:54863, Jan 17 2008, 09:20:17) 
[GCC 4.1.2 (Gentoo 4.1.2 p1.0.2)] on linux2

Python 2.5.1 (r251:54863, Oct  5 2007, 13:36:32)
[GCC 4.1.3 20070929 (prerelease) (Ubuntu 4.1.2-16ubuntu2)] on linux2

Python 2.5.1 (r251:54863, Feb 18 2008, 13:35:09)
[GCC 4.2.1 20070719  [FreeBSD]] on freebsd7

This is (currently) the intended behaviour.  The rfc822 module has an
__all__ attribute that lists its public functions and classes, so "pydoc
rfc822" only shows these things.  formatdate is not listed in __all__.

If you'd like to discuss ideas for changing this behaviour, possible
forums would be comp.lang.python or the python-dev list.