Security :: Extending Asp Forms Auth To Be Multi Tenant / Domain?

I am developing a web application which implements membership, roles and profiles. I also want it to be multi-tenant / multi-domain. By this I mean that I will have hundreds of domain names pointing at the same web application. I want users to be able to create accounts at each individual domain. I want the opposite of "single sign on". I want users to be able to use the same username, email address and password (or different ones, at their choice) to create accounts at the different sites. I do not want users to have any awareness that the different domains have any relation to each other as they will look different and have different content. The first problem I run into is that usernames have to be unique within the forms auth aspnet db.

Well, if they want to use the same username to register on 10 different sites then I need to support that. My first thought was that instead of identifying a user by their username, I need to extend the aspnet tables and sprocs to consider the username + a unique site identifier. Is there any sample code or has anyone else ever done this before? Should I just abandon the built in aspnet forms auth and roll my own custom auth scheme? The data on the sites is not sensitive so security isn't really that important to me.