True. Now that you mention it. I seem to remember having iMessage (the feature) be opt-in in Europe. So the feature is on by default in the US for sure? I guess that somehow negates my earlier post. But I suppose people would notice the green bubbles everywhere and remember to turn it on.

Hmm is this a defense against antitrust concerns as well?

iMessage is not opt-in. It's enabled by default even here in the UK. The good thing about it is that it doesn't even require an account, if you just want to use it with a phone number (you need an account if you want to use it with another identity like an email address).