I'm planning to use a Thinkpad T23 laptop for my car-PC and I'm currently working on the automatic startup/shutdown. My plan is to buy a 12V car adapter for the laptop and connect that to the ignition. That way, the laptop will have power only when the ignition is on. Then I will configure XP to standby once the ignition turns off and the laptop goes to battery power (can be done by configuring the XP power settings). However the big kahuna has been how to make the laptop automatically resume upon ignite. So far, WOL (Wake-On-Link) seem to be the way to go. WOL will resume the system from standby when the configured network interface has link. To test this, I made a small loopback adapter consisting of a RJ45 plug where I connect pin1 to pin3 and pin2 to pin6. Then I enabled WOL on the NIC (Network Interface Card) (the advanced tab on the NIC configuration dialogue accessed from the network connection properties) and marked the box to allow that NIC to resume the system from standby (done under power settings in the NIC configuration dialogue).
I tested this on an IBM Thinkpad T20 (3Com 10/100 Mini-PCI NIC) running W2K and it worked flawlessly from standby. Couldn't test hibernate though as the PC was running PointSec software that is incompatible with hibernate functionality.
I then tried this on a Thinkpad T23 (Intel Pro 10/100 VE NIC) running XPPro but that did not work. At first, the NIC driver did not support WOL so I downloaded the latest version of the driver from Intel. XP would not accept that driver automatically so I had to force it. After a reboot, I had the Wakeup settings under the NIC properties so I set it as on the T20 laptop but this time no go. The laptop would not resume when I inserted my loopback adapter in the NIC.
I attribute this purely to software issues. The BIOS power settings on the laptop are only applicable when the laptop is running APM software. Microsoft W2K and XP are using ACPI where all powersettings are software configurable, thus rendering the BIOS power settings mute. So wether or not WOL works is a matter of NIC driver functionality and so far it seems that 3Com drivers support this feature and Intel drivers do not (although it looks like they should).
If I get it to work, it's a small thing to take two relays, controlled by the ignition switch, to bridge those four pins in the NIC slot to resume the laptop from standby. Haven't had the possibility to try it in hibernate mode but I guess as it is OS controlled it will not work because in hibernation the laptop is completely shutdown and thus, any OS functions will not run.
It would be nice to know if anyone else has fiddled with this and actually got it to work (with an Intel NIC).