With Apple's announcement of the new iMessage, mobile instant messaging appears to be a new battleground for differentiation between smart phone platforms.
Blackberry was the first to do this with Blackberry Messenger. BBM is a great tool in that it is fast, works really well, and is free. But of course, both parties must have Blackberries. iMessage is seemingly similar from a functionality perspective but adds the usual Apple flair with a nice UI. Apple has also integrated SMS into iMessage, so at least if you need to contact someone who isn't using it, you don't need a separate app to fall back on SMS like you do on Blackberry.
Rumor has it Google also has something in the works; whether this is an enhanced version of Google Talk or something else remains to be seen. However, since every smartphone platform has a more or less similar mobile instant messaging service, how does this differentiate or give one platform the edge over another? It will never happen - but if BBM could talk to iMessage could talk to Google - that would be the true killer of SMS and really drive up adoption and usage.
SMS works across any phone - even non-smartphones of course - but carriers use it as a cash cow. AT&T even dropped their cheapest SMS plan recently, making the monthly cost $10 at a minimum. It reminds me of how PRODIGY used to charge per email message in the 1990's. You got 30 for free, and then had to pay per message after that. We haven't come far in 15 years in terms of billing or service models apparently.
I have used things like Kik messenger, which running on iOS and Android, is fairly close to a true cross-platform solution. But even Kik should be integrated with the others. Imagine if you could only email other users of the same email platform as you. "Oh, you use Outlook? I'm on Gmail. Sorry!" It would never fly. Why do we have to put up with it in text messaging?