Can you really scale up with Django...given that you can only use one database? (In the models.py and settings.py)
Read scaling to millions of users is not a database problem, but is fixed with load balancing and caching, etc, see S. Lott above.
Write scaling can indeed be a database problem. "Sharding" and having multiple databases can be one solution, but that's hard with SQL while still retaining the relationality of the database. Popular solutions there are the new types of "nosql" databases. But if you really have those problems, then you need serious expert help, not just answers from dudes Stackoverflow. :)
Django only allows you to use one database in settings.py.
Does that prevent you from scaling up? (millions of users)