If you read the HIG, Apple wants devs to use the splash page to show a picture of the app's UI elements--just with not text. All of Apple's apps do this--if you open up the address book, for example, it'll show you a picture of an empty UITableView and such. And once the app loads peoples' names will appear in it.
They specifically recommend against using your logo as a splash page, but most people do it anyway.
There are good reasons to use a splash page rather than a "fake UI" image. For single-purpose apps (say, Calculator, or Weather) the state of the UI post-launch is easily predictable, so you can actually make a splash image that lines up with what the user sees after.
For more complex apps, especially apps that maintain state, this becomes impossible. You can make your splash image your "home" view controller, but if the app is launched from a URL (a large use case for many apps) the user will see an "empty UI" that is not what they're looking for, suddenly replaced by UI they are looking for.
Making matters a bit worse is that, at least up until iOS6, switching between apps (not just a cold launch) can also show the splash image briefly - but your app snapshot remains in memory, so you cannot guarantee the "empty UI" is what your user actually ends up looking at.
Splash images suck, but for a lot of apps they are the least of several evils.
I actually really, really hate that concept and I wish Apple didn't try to get people to use it. Numerous times I've tried to interact with a UI that is actually just a flat PNG image.
It feels like Apple is advocating using a really ugly hack to make it look like app loading times are shorter than they really are.
They specifically recommend against using your logo as a splash page, but most people do it anyway.