Will mobile Twitter apps mean a decline in SMS?

As I began yet another tweet from my iPhone, the thought struck me - I wonder if there'll be a decline in SMS? Surely SMS is what Twitter was modelled on in the first place given its message limit?

I also remembered the reports suggesting that traditional email traffic was seeing a decline as Facebook users began utilising FB mail. I wonder if the same is true of the IM services?

I wonder if the Mobile Marketing Association (MMA) have spotted any trends since Twitter growth spurt?

What are your thoughts?