Perhaps the Dual Shock 3 Sixaxis controller will be unveiled at TGS, perhaps it won't. But when in doubt, turn to the United States Patent Office.
GamesRadar did a little digging and has turned up the following information plus the pic you see here. It seems Sony won't be limiting itself to just force feedback: Sony filed a patent with the U.S. Patent Office back in September of 2006, and it seems examples of both "voice active consoles" and "pressure-sensitive buttons" were included in the documentation. The voice active consoles apparently hints at microphones included in a console itself, which would feasibly allow you to record your own voice into the machine and perhaps match it with the character on screen.
Pressure-sensitive buttons aren't a revolutionary theory; the Dual Shock's buttons were pressure sensitive, to some extent. But in this design, Sony would include three distinct stages of input…whatever that means. There's a lot to consider, though, and you may want to peruse the following quote:
"The input device of claim 4, wherein the first, second, and third control signals are proportional to the amount of pressure detected along the associated first, second, and third axes."
Maybe you can make more of that than we can, but we're not going to try. All we can say for sure is that Sony may intend to do a lot more with the Sixaxis than merely add some last-gen rumble technology. …that last one's for you, Phil Harrison.