I just thought I'd share something.
After verifying my site with google.com/webmasters/tools, I saw that one of my top queries to find my site (where apparently I'm at position #18 for) is:
inurl:wp-signup.php "powered by wordpress mu"
By doing this search, spam bots can identify our blogs and go to work signing up.
Although I don't allow new users to create blogs, I don't want to over run my member list with a bunch of auto-generated users. Guess it's time to implement this plugin!