When I load my game on iphone it shows a black screen for five whole seconds, after the Unity logo, and before the game starts. Has anyone else had a problem like this?
It only happens on the iphone.
A loading issue maybe? iPhone’s seem to be slower then the iPod Touches.
I read that unity has slow loading for the first frame but after that it’s ok, try making the game first of all load an empty scene and then load your level, I can’t help much since I don’t have unity yet 
It happens to me but I’m using Resources.Load function in the Start (or Awake) function of my first scene. So it’s normal for me.
What do you have in your Start/Awake functions?
Ah, I think I found out the root of my problem. I didn’t have my textures set as compressed in the Build Settings, so they must have been taking forever to load, or just getting confused. Now that I have that checked it works fine. 
Is it for your Tumbledrop port to iPhone? Tumbledrop have very clean 2d art, did you managed to get it worked with compressed textures?
The only way I’m satisfied with compressed textures is when I use a scalled down version of them (256256 compressed texture for a small 150150 2d element).