I heard somewhere AT&T asked Apple not to approve it because they don't want people using it over 3G. So Sling Media may have to go back and rework it to be wi-fi only before it'll be on the app store.
Because there are a lot more iPhones out there. That would be a huge bandwidth usage to allow iPhone to use a Slingbox over 3G. Plus, they can control what goes through the App store, so they actually get to have a say. Of course it will work, but AT&T doesn't want that much data going over their 3G network.