That's a great way to build up buzz, but at the same time it makes for a pretty lame event. My guess is we'll see a few new apps or features that weren't baked into the employee phones (yet).
Apple is the standard bearer for message control, I don't know that Google is any good at stopping internal leaks - it seems like we know a ton more about the Nexus One than almost any Apple product announcement already.
As far as Google's leak control, I think that they can be secretive when they want to be. Part of that comes from the fact that they have so much going on — you hear a rumor and it could well be related to some random research project or internal test, etc. But I've been to a few Google events where there were product launches that hadn't been previously rumored.
A good example is probably the Chrome launch. As far as I can remember, there wasn't any significant buildup in rumors of a "Google Browser" in the weeks before that announcement. Granted, the cat came out of the bag when a mailing company accidentally shipped the Chrome comics a day early, but that didn't come from an employee leak.
Really, I'd love to see the big surprise be a data-only plan option.