I've been playing with mig33, a combined IM, SMS and VoIP application with more than six million users worldwide in 200 countries.
I think you haven't heard of it yet. But with a push toward visibility in the U.S. I recommend that you should. At least try it out.
They have a new web-based interface that I personally think is one of the most attractive, clear and iterative I've ever seen. And if you come here often, you'll know I don't say "ever" all that often.
You haven't heard the last from me re: mig33. They are sending me a phone. When I get it, wait and see how fast I deploy it, test it and report back to y0u.