I did some extensive testing with a high accuracy current meter, and have determined that the Carnetix P2140 draws about 40ma (at 12.5v) when it is in shutdown mode. i.e. when the ignition is off and the PC has shut off, it still pulls 40ma.
That's about the same draw as the rest of my car, so having the P2140 in the car doubled my car-off power draw.

Since my battery was getting weak after several days of being off, I decided to fix it.

This is what I had:
Battery -> 15A fuse -> P2140 power in.
Ignition -> P2140 sense

What I really wanted was for a relay to cut power to the P2140, after the computer had already shut down (since I'm not using hibernate or sleep). So this is what I changed to:
Battery -> 15A fuse -> Relay -> power in
Ignition -> P2140 sense

The interesting part is what powers the relay. I setup two 1A diodes that feed the relay, and wired one to Ignition, and the other to the 12v output of the P2140.
That ensures that the relay is energized if the car is on, or if the PC is on. Since the P2140 shuts down the PC when the car goes off, the second diode to the relay from the P2140 holds the power on, until the P2140 shuts down too, which opens the relay and ensures that the P2140 doesn't have any phantom power draw.

Of course, this means I can't use hibernate or sleep. I could never get hibernate to work well- my system takes 90s to recover from hibernate, and 45s to cold boot.
Sleep may be missed- it was nice to go from key-on to music in less than 5s. But my continually run down battery convinced me that I can live without sleep mode for now. (The P2140 did not dead-man timeout reliably, sometimes it just ran the battery in to the ground. I had to replace the car battery once already due to the damage...)

Some day I may put in small battery to keep sleep active so I can use it again, but then I'd have to deal with all the "devices don't come out of sleep perfectly" issues again.

Hope that helps someone-