Wrap unity3d inside an existing app

Could you clarify this?

What do you mean by "Unity 3D"? The Unity 3D editor? A finished Unity 3D game or project?

And what do you mean by app? What platform? Windows 8? iPhone/Droid app? Web app?

So far I've managed to create an iOS library from the angrybots unity sample game (in the future I'll replace it with my own game) and load it into another iOS app. There are several problems:

 It needs its own view controller & view (that is doable, with a couple of hours I'll manage to get my own EAGLView updated by a controller)
 Capture the surface touch and send it to the native unity layer (again doable)
 Start and stop the unity game - once the game is started, stopping it and starting it leads to app crashes
 Start multiple games, each one in its own window - if 3) fails i do not have high hopes on this one
 Start the game from a UI Button, fullscreen, and tapping on a ui button inside the game get back to the main app while the game itself gets paused (I was thinking to use url schemes in order to get back to the spawning app but there should be another way - how can i get a message from the objective c code that the game wants to be paused/terminated?)

What are the caveats for doing something like this with a unity game?