Yes, I already found that while studying the architecture of DNN yesterday night...
And I logged in today to convey this to other users, but Cathal has already done this... Still, I would like to put in in my words as follows:
"DNN has a built in-assumption, that whenever there is an attempt to register with the same username in 2 or more portals of a single DNN installation, the end-user trying to do so is the same. Owing to this, the login credentials across these multiple portals are maintained uniquely. Thus, changing them at one portal automatically gets reflected in the other portal. However, the Profile data of a user is maintained per portal. So, this same user can have differing profiles across portals. This is even desirable as the profile properties for each portal are set independently by the profile admin."
My analysis tells me that this behaviour of DNN has resulted from an attempt to being integarted with Microsoft Membership API, which does not support virtualization within a single ASP.NET application (i.e. separate & independent sets of users within a single application install).
In some way, this makes sense when the portals are separate services from a single provider (like Orkut, Gmail, Calendar etc. from Google).
However, for hosts looking to accomodate multiple clients in a single installation, this is troublesome.
And that is exactly what I am also looking for. And I have also found a way out. I have decided to create & use my own registration form for users. And I am going to make a user's e-mail id his/her username. That would only require me to create two controls, one for creating users, the other for allowing them to change their credentials. Except fot that, the concept can entirely integrate with DNN. Thus, if a user uses same e-mail id to register on different portals hosted together, that automatically means that the user is the same, satisfying the DNN assumption.
However, I wold like the DNN team to underline this assumption more explicitly in their documentation, and books like Professional DNN that are from the same team.