Essentially, “This is happening in a few weeks, leave your mail, we won’t spam you, and we’ll tell you about it.”
I submit my email.
Then the page morphs and thanks you for your interest. (Not altogether different from Lazy Registration.)
And if Flock is different to the last one I tried, they’ll actually send something back.
Update: They’ve auto-mailed me an invite to join a mailing list. I guess they can’t do RSS due to auth issues, but I’m going to skip on the mailing list offer.