if (top.location != self.location) top.location.replace(self.location);
This blog post at tomuse.com argues DiggBar, Facebook Bar, and ow.ly top bars steal traffic, steal links, steal content, and steal potential revenue.
It's hard to disagree. While they provide some amount of value (being able to comment back on Facebook is kind of useful) -- it's kind of an annoying distribution hack too. Why would I want my browsing to carry around this annoying parasite of UI? And if I'm a site owner, I wouldn't like that either.