The problem with all these 3rd party systems. GTalk,whatsapp,KIK, and so on, is that they aren't integrated into the main SMS system. It's a different app. That's my issue. I don't want a different app. It's one of the best things about Apples iMessage.
This might catch on with the younger crowd, but for people who prefer efficiency, it's just one more thing to keep track of.