Be prepared for HOURS and HOURs of development time to get the site up and running. It's not a plug and forget app, but it will suit your needs.
read the forums. edit your themes for all your users, optimize your home page for new user subscriptions.
read the forums.
You MUST have VHOST enabled in Apache if you want to use user.yourdomin.com for new accounts. Strongly reccomended.
read the forums
Did I mention to read the wpmu forums?
It's very young and in it's infancy. There isn't a lot of support, but there are knowleable people around on the M forums that will get you through the rough patches.
read the forums
This isn't for the faint at heart, but as mentioned, it will suit your needs perfectly.
There really isn't anything else out there that I know of that will do what your asking, although I'm sure if you look hard enough, you'll find something.
Since your familiar with WP, WPMU would be he next logical step.
I'm assuming your a developer, so I don't think you will have much of a learning curve, you just need development time. Itwon't be up and running over night, since you will need to modify/test most the themes (read the forums for MU specific themes).
You will also wan to read the forums for WPMU widgets. Most WP widgets will work, but not all. Some need modification, usually only a one or two line adjustment.
just check the forums.
Separating WP, from WPMU administration is a little confusing at first, but you'll get a handle on it.
So just head over to http://mu.wordpress.org and read the forums.