Kotaku managed to get a quote out of Sony's Shuhei Yoshida as to why the hardware wasn't shown at the PlayStation Meeting last night.
"As far as the DualShock 4 is concerned we wanted to do the live demo. And we were like, yeah, we have to show the controller, because it's awkward not to show the controller when someone is doing a demo. And also the feature that the DualShock 4 has, the share button, is a very key idea behind the design of the PS4... We really wanted to explain what we've done with the DualShock 4, but as far as the system itself we have to keep something new for later. Otherwise you'd get bored."