The clever thing with iMessage in particular is that at first glance, it IS still an SMS you're sending - it just takes a slightly different route which avoids operator charges.
This is what WP7 did a while before iMessage came out; you can switch messaging mode to send SMS, Live Messenger or Facebook messages, so all conversations are interleaved and look the same. If the contact is on FB/Live Messenger then you can send via those technologies, and WP->WP messaging can happen this way as well. It's cool because it all looks like one chat history with the person, and it'll let you know if they're offline on a certain service. You can still send messages to offline people, though.
I imagine they'll try and build in Skype messaging as well, if they haven't already, and there are various requests to open up the API completely so WhatsApp/Kik/whatever can also join the party. That'd be ace.