I would recommend that you look into something simliar to what David is talking about as per above.  I think while the multiple portal alias route will work, you're heading down the wrong path as it leaves you little scope for customisation and unique content.  You may think that they can all have the same website, but websites are personal things and people want to start fiddling around with it pretty much as soon as they get one.  If you don't have this in your plan you might have trouble.

THe other problem with your plan is that if you are planning on getting search traffic, you're going to fall foul of 'duplicate content' penalties in search engines.  Basically what this means is that search engines are going to see the same content, same IP address and multiple domains and conclude the information is not unique and therefore not useful, and therefore rank it poorly, or, in the extreme case, decide it is spam and exclude it from the index altogether.  If you're not looking for search traffic, no problems, but if you are, then you need to think it through more carefully from the beginning.  The content module David suggests above may be the sort of thing to do.