I write for iPhone Life magazine, and we get to beta test apps before they reach the market.
However, many times they keep the info under embargo... in other words, we can beta test the app and work on a review or an article but can't publish anything until the app is released. There are some cool apps I can't talk about yet, but when those developers give me the go ahead, they should be newsworthy.
When you consider that Apple's approval schedule is, to put it mildly, unpredictable, tipping one's hand too early could give a competitor time to develop a new (or modify an existing) app with similar features. And since an update is easier to get approved, it could actually come out before the new app!
So I think some developers will be hesitant to promote apps that aren't available. It would be a different story if they could control the release date.