When to create a new app (with startapp) in Django?
I tend to create new applications for each logically separate set of models. e.g.:

User Profiles
Forum Posts
Blog posts
I've googled around for this, but I still have trouble relating to what Django defines as "apps". 
Should I create a new app for each piece of functionality in a site, even though it uses models from the main project? 
Do you guys have good rule of thumb of when to split off a new app, and when to keep functionality together with the "main project" or other apps?