Adam - Perhaps the message travles just as fast, but I think that Massimo's point is more cultural.
When you receive an SMS there is no underlying assumption of the sender that you reply immidiatly. It's not a chat. That's how SMSs always been.
Whereas, when you're in an IM session, the sender might get "offended" if you delay your reply too long (That's how Pc based IM always been). It has to be interactive.
It could be, though, that people would learn to adjust their cultural expectations to the technology... so the IMer would understand the other side is actually IMing from his mobile and cannot reply as quickly or as promptly.