I just connected a regular Bosch auto relay to my power switch after I realized that if I press and hold down the power button of my eee pc, it has no problems starting up and staying on. I read others have just jammed their buttons mechanically in their battery-less old laptops so that it turns on when it receives AC power.
I connected the leads from my laptop's power switch contacts to pins 30 and 87 of the relay. I had soldered a headphone jack for this.
Then I connected 86 to an ignition-switched fuse line and 85 to ground.
Not as elegant as a momentary switch, but it's cheap, compact, and works!