Best approach for multiple sites in a single deployment?

I would like to create a subdomain based authentication system, like the one on 37signals, freshbooks, codebase, etc. The objective is to do something like smaller but in far smaller scale (I suppose a maximum of 100-1000 blogs). All other models (blog posts, comments, etc) already have a foreign key to Site. 

1. How can I allow different users to have the same username in different sites? How can I make so that when a user logs in, he logged in to a specific site (logging on blog A does not log you to blog B).

For allowing the same username in different sites I guess I'd have to write my own authentication backend and a new model for User, as I don't see how the default User model would fit. Is this a good approach? 

2. Is there anyway to make the sites framework fetch the current site based on the subdomain instead of the settings? Or am I seeing this the wrong way?

My plan was to use a single deployment for everything and use the sites framework, but as far as I've understood, the sites framework relies on the settings.SITE_ID parameter, which would be the same for a single deployment. What would be a good approach here? Creating a new settings file for each new blog?

Many thanks in advance!

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