Automatic power on/off

check out my wiring diagram, i think itll accomplish exactly what you're looking to do.

viewtopic.php?f=35&t=9686

Relevant to this topic:

viewtopic.php?f=63&t=6050&start=75

2 things of note:

RPi uses ~0.12A at sleep mode, around 0.47A at peak usage (depends on peripherals and such).

Secondly, Dom is considering a GPIO switch for auto sleep/wake!

In theory yes. Trouble is we are almost stateless at this point. I sleep before sdram has been enabled, and this is just after a reset.
It turns out there is a register (the one written to in the kernel power down function) that does maintain a few bits of state over the reset, so I can distiguish a power on/reboot from a halt.

There's not enough information to set a wakeup alarm duration, or GPIO interrupt pin.

Possibly we could say, a rising ede on GPIOn (for a fixed n) wakes us up again. (Maybe even the HDMI hotplug line, so you could possibly provoke it by powering on your TV).
Although whatever you choose, someone will grumble...