Is there some special hardware that was added to 3GS that makes it so much more special for vocal commands ie voice dialing, that the 3G doesn't have? Or is this purely a marketing strategy?
The whole point of marketing is to get people to buy a new phone. So even if they could just update the 3G to do it, they will market it as a new feature in the 3GS only to get more people to buy it.