It will be similar to the iPod interface on the iPhone. Simply have a 'more' option where you can have more apps and manage the apps that are on the first screen.
One interesting note is that the pamphlet that you get at the AT&T store shows the iPhone without the YouTube app icon. Of course YouTube was added pretty late in the game so it's not surprising that they would have an old image. But think about it, is YouTube a must have on the main screen? Probably not but there was room so cool go for it. I think there will be an SDK at some point in the next 6 months that allows developers to create apps for the iPhone and possibly sell them through the iTunes store. Once synced to the phone you'll be able to set whatever apps you want on the main screen.