I have read the excellent Spencer Harbar article on multi tenancy.
http://www.harbar.net/articles/sp2010mt1.aspx
Makes perfect sense and I can see how the service applications can be partitioned between tenants.
It seems, however, to completely ignore identity / authentication.
Since this is configured at the web application level this infers that having spent time segregating all the service applications all tenants must share the same authentication mechanism. So if FBA was being used all user names and passwords in the same database.
Additionally the people picker segregration uses an active directory organisational unit which infers use of AD and integrated windows authentication whereas the concept of hosting for multiple tenants suggests otherwise.