I'm trying to plan a mac mini integration into a toyota prius. you can search mac mini prius for several projects.

the prius has the perfect slot for a mini and the built in lcd screen works well as a monitor solution.

so my question is...is there a way to hibernate a mac mini so when i turn the car on it will resume like a laptop does whenever it loses all battery life while asleep.

Yeah, you should be able to get a DC to DC converter that basically hooks right up to the battery. Like an oversized laptop battery you can put it to sleep and wake it just like would at home. Down side to this that I've heard is if you aren't going to be driving the car for a while (two or three days in worst cases) it drains the battery down to far so the car doesn't start. It would just be best to shut down the computer every night and start it up every morning, I know I'd be too lazy for that one. best of luck.

yea thats a solution, but i meant this scenario

you put your macbook to sleep...it runs out of batteries when asleep...mac os x has saved all RAM to the HD and in the event that the battery runs out it can still awaken itself in the exact state where it was left. It doesn't actually consume any battery when in hibernation.

i need to be able to trick a desktop into going into hybernation.

So, if I understand correctly, this is what you want to happen:

1.) When you start the car, the Mac mini either starts up or wakes from sleep/hibernation.
2.) When you turn the ignition all the way off, you want the Mac mini to go into hibernate mode.

This sounds doable. While I can't help you with the fancy combination of start/wake and off/hibernate, I can help with the hibernation itself. Check out SmartSleep. It's a preference pane GUI for the hibernate command-line settings. You can use it to set the computer to always hibernate (instead of sleeping). This means that when you tell the Mac mini to sleep, it will hibernate instead. No "tricking" necessary. Just some changing of settings.

Before you use SmartSleep, you'll need to run this set of commands in Terminal:
sudo nvram nvramrc='" /" select-dev
" msh" encode-string " has-safe-sleep" property
unselect
'
sudo nvram "use-nvramrc?"=true

I suggest you restart, then install the PrefPane, then see if it works. If not... Well, it should have worked.