Couldn't find this covered on the forums so I wanted to give you guys a heads up. It appears that Sploggers are finding the WPMU sign up pages by this query:
inurl:wp-signup.php +intext:"gimme a blog"
Five visits directly from this query over the last 48 hours. I've changed the wp-signup.php to read "give me a blog" - we'll see how it works once the big G updates its cache of the page.