I'm assuming you're not using (or talking about) the splash images that you set up in xCode. Is that correct?
We're running some beta tests this weekend with my new loading image (we currently use splash screens). I'll let you know if we run into any issues on the iPhone 4. I've been really unimpressed by the performance of AppGameKit on the iPhone 4 and 3GS, the frame rate is low and all over the place.
Does your game freeze at the black screen or does it load after a couple of seconds? On Android we were having freeze up issues at app launch, but after moving o the prebuilt interpreter we've not seen these issues. But that wouldn't explain the issue on iOS.