I'm going to build a turn-key solution for a vertical market, and would like to offer both options: software as a service, and give them the opportunity to host the application on their own. In other words, I'm aiming to have similar deployment options as Joel's FogBugz.
I'm a Python programmer, and I could fly over the project with Django. There are several reasons I prefer PHP though:
1) Django installation, and configuration assumes you have access to a shell (my target is not the programmer type). Although I could offer installation service, but not on their servers.
2) Django runs only on some specific hosts that must take special care to enable it. Installing mod_python/mod_wsgi, and most likely the minority of my potential clients would have root access, or even a cpanel.
3) Using PHP would mean I could run it on their existing server. I would have no need to move them to a Django-enabled server, and no downtime for their emails, while the DNS updates.
On the other hand, I have very little experience with PHP. Smarty as a templating language looks nice, and works similarly to Django templates. It doesn't offer template inheritance though, except in a very hackish way in which I wish not to use as it could break the application if the designer messes them up. What do you think? Thanks in advance!
Deployment is clearly a problem for all non-PHP based web apps, but I think things are getting better with the DreamHost/Engineyard type ISP's who provide Ruby/Python etc. out of the box.  It also looks like there's going to be a lot of discussion at PyCon this week about ways to fix deployment problems.  The growth in popularity of Django, Turbogears, and Pylons is driving demand for better deployment solutions.
That said, if your target market are people hosting on the very low end $12 a year type ISP's then I don't think you have much choice other than PHP.
Finally, one thing I disagree with you is running PHP and Django on the same server.  I'm running a few PHP apps on my server with Apache and dozens of Django sites with mod_wsgi in daemon mode.  Running it that way means the Python interpreter doesn't use up ram in the Apache workers and vice versa, the PHP interpreter isn't contaminating my mod_wsgi daemons :)
Django or CodeIgniter for Turn-Key Web Application