Here's how my Pre2 behaves on the touchstone:
If it is asleep when I place it on the touchstone, it immediately goes into exhibition mode. This brings up either the blurry grass scene or the black background. If I tap the screen, it switches between the two. If I tap the gesture area, it awakens the phone.
To get back to exhibition, hit the power button.
TIP: To turn the screen off completely while on the touchstone, press the power button while in exhibition mode. Then the phone is completely dark (except for any notifications that come in, which you can turn off in the relevant apps).
Incidentally, this works on the Touchpad too.