With my C760 I could just remove the battery and reinsert it and the on/off button would work again.
When you hit the on/off buttom regularly the Z is actually in hybernation mode, that is why the Z power up again so quiclky after you hit the power button.
However when you "really" shutdown, the power is completely turned off and as InSearchOf mentioned there is no real power button to turn the Z back on.
IMHO there is no benefit in halting your Zaurus, there is a huge negative, the 2-3 minute boot time when it resets.
Stu