The readline library supplied in OS X 10.6 looks good enough to use in 
Python.  It would be nice to enable building with this library, to avoid 
having to install GNU readline.

There's a curious off-by-one difference between Apple's readline (which, 
as I understand it, is just libedit wrapped to look like libreadline) 
and GNU readline:  with 'n' history items, the valid indices for Apple's 
readline are 0 through n-1;  for GNU they're 1 through n.

I was able to get Python trunk + system readline working on OS X 10.6 
using the attached patch (which obviously isn't suitable for applying, 
since it breaks the build with a non-system readline).  A side effect of 
the patch is that readline.get_history_item and friends store the first 
history entry with index 0 rather than 1:

Python 2.7a0 (trunk:74735M, Sep  9 2009, 19:40:25) 
[GCC 4.2.1 (Apple Inc. build 5646)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> import readline
[39474 refs]
>>> readline.get_history_item(0)
'import readline'
[39476 refs]
>>> readline.get_history_item(2)
'readline.get_history_item(2)'
[39476 refs]

Interestingly, the Apple-supplied Python also behaves this way:

Mark-Dickinsons-MacBook-Pro:trunk dickinsm$ python
Python 2.6.1 (r261:67515, Jul  7 2009, 23:51:51) 
[GCC 4.2.1 (Apple Inc. build 5646)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> import readline
>>> readline.get_history_item(0)
'import readline'
>>> readline.get_history_item(2)
'readline.get_history_item(2)'

If people think this is worth pursuing, I'll put together a proper 
patch.