Oh, cool. Someone read my blog ;)

You can set the registration up so that it is on different tabs for each site.

Let me know when your ready to do that.  I would also expect that you might want to put people into a different "registered user" role depending on what site they sign up on, but it would only take a little code to be able to do that also.

I know of someone else that is doing doing this for a school district so that all the schools have their own portal but they share the same userbase.

I'll be helping her if I can, so let me know if you need any assistance on the Registration and maybe we can get a working solution for everyone.

Thanks for the offer of help.  I finally got a bit of time to work on my sites tonight, and I'm ready to figure out how to get the registration on different tabs for each site.  I'm going away after this weekend for 1 week and a half so my time on this will be spotty, but I should have some time this weekend if you can give me any pointers.