Huge pre-release hype has definitely been shown to be a reliable non-pattern for success of websites.
Yeah, interesting, that one. Wonder why. Perhaps because it costs a lot to do PR on this scale, and there are two routes to that kind of funding: to have built something that already works (Google, Facebook, Delicious), or to have high-profile founders and grand-sounding plans (Flock, Mahalo, Powerset). So a startup with lots of PR but no product must be the second type.
The PR is so damn good it will probably cause expectations to be completely out of whack (usually a problem startups do not have to worry about).
Why do you think Powerset's pr people are able to get so much and such positive press.