Seems like the 'powercfg' -lastwake, command should be able to help you here.
Though waking from hibernate is more of a hardware thing than a software one.
I got fed up with having a home-made machine that wouldn't stay asleep so I eventually went and bought a pre-built 'designed for Windows 7' machine that goes to sleep, stays asleep, and wakes up to record programs with Media Center properly.
I don't like doing things like that, the other machine worked well enought otherwise and there has to be a reason behind it waking up, but finding out what it is doesn't seem to be very straight-forward.
