I'm curious about this too. At least now I have fairly decent confidence that when I send an iMessage to someone, Apple protects their identity to whatever standard they have. Whatever trust I put in Apple, at least it's a single point of failure.

What happens if interoperability is enforced and messages have to be end-to-end encrypted? Wouldn't that mean that any side-loaded Android app would have to be able to get hold of my friend's private iMessage key?

On iOS I guess you could still keep the key private through Apple's SDK, but what about other platforms?