I tried it with a very customized and tuned-up WPMU version and it worked amazingly well, but I don't know if it was because of the subdomain thing or the ultra-customization (which was very oriented to SEO).
Now, this is not a fact, so take it with a grain of salt, but I have this theory: Google adds a slight bit of pizazz to subdomains since they're usually used in free host environments. Maybe not much, just a little something. If this is true, then you could build your site with subdomains and they'll add a little to your main page. Just to reinforce that, maybe you could have a working signup page that Google can follow but you set it as non automatic (meaning you have to manually approve users, which you won't)
The above being said, this gave me an idea for a tutorial based on past experiences and how to get extra traffic for free, be tuned for it
