Is it possible, and/or advisable to develop Django web applications on OS X (10.6.4 and 10.5.8) using Python 2.6.5 64-bit? Why?
It certainly is possible: I do it every day.
Some tips: 

use virtualenv to sandbox your python packages between projects.
use mod_passenger (via Passenger.prefpane) to make VirtualHosts easier to deal with.

You may need to fiddle a bit harder with things if you run stuff under mod_python, as I recall having to work hard to get a version compiled that worked with the version of apache that is installed by default, and the python I was using.
I'm trying to decide on which architecture to choose for developing Django 1.0.x through Django 1.2.1. I've managed to get MySQL, MySQLdb, PIL, and Python 2.65 installed on Snow Leopard using x86 64-bit builds, but I'm curious as to whether or not there is a definitive answer to this question at the moment, and if so, why?
Thank you!
Michaux