BuddyPress.org

Oh man… that sounds like a mammoth of a project. Here’s the dealbreaker, though — and I speak from personal experience — if you’ve already got something else running in your root directory, it’s not gonna work. And it actually says so in the WPMU readme file…

“If you want subdomain blogs, you must use the root of your site.”

Assuming you didn’t have anything installed in your root, I’m pretty sure there’s no plugin/patch job that exists yet to hook up multiple installations of BuddyPress so that a single user could hop around from site to site. I suppose if they registered every time with the same username/password, they could just rely on the “saved password” feature most browsers already have — but that’s probably not what you’re looking for.