iPads use extremely efficient hardware.
I bet while it's in sleep it's only connected by wifi to the push notification system, which alerts it in whatever fashion to "wake" and ring, but yeah, stupidly efficient. It's why Apple chose to do a lot of things, including the centralised push notification system, the way they do "multitasking" etc...it's all in the name of better battery life.
Inherently, the iPad is never really asleep fully, whereas a Mac will go to sleep.
It's an interesting notion to have a MAc do what an iPad does, though.
Incidentally, there are certain channels that ARE available to a Mac while asleep. For example, if you router supports it, it can actually wake on LAN activity (used for "Back to my MAc", for instance). And obviously thins like Bluetooth can still be on, as bluetooth mice clicks will wake a mac. These are probably exceptions, however - not sure if you can connect those services with FaceTime activity.
I also remember that before Lion, you could wake a mac by moving the trackpad, which no longer works. Or was that just for the display?