BSR I would also be willing to share a DS10A with you. I also, do not have an extra W800 however to offer.
The bottom line though, with a DS10A and a 12v relay, I wouldnt think that too much else would be required for setup. Tied into a realy with the ignition, or with the brake lights as Electron suggested should be all that would be required.
With the brake lights, it really would not know when the car was home or leaving, without a creative bit of scripting. As when you let off the brake an off would be sent. Also the brake can be pressed multiple times when either comming home or leaving.
With the ignition being used, when the car is started, it would simply send an on signal, and an off when the car is turned off. As a fallback, the DS10A will send out the status every..... 56 minutes, I beleive someone said, thus "pinging" the vehicle.
As for problems with a car starting, such as a cold morning or a clasic car, the easy answer would be a simple condition in the event to not retrigger for 5 minutes.
To make this really rock solid, it could be set up that when an on or off is seen, to set a virtual device to this value every time that it is seen. Then 5 minutes later check to see if it is set to on or off. If it is set as on, then the car sucessfully was started. If it was set to off, then the car was attempted to be started, but didnt work. (Have HomeSeer call a Taxi, your spouse or AAA for you at this point

) Or if another vechile is "sensed" as being home, have the whole house announce that you need a ride hehehehe.
An easier way is to just set a virtual device with whatever the last command was. Say you tried to start the car 3 times and it then started (on,off,on,off,on), the last command seen was an on, meaning the car was started successfully. If you try multiple times and give up (on,off,on,off,on,off), the last command seen was an off, and you know the car is still at home, as it couldnt leave with the ignition off.
Getting a little more creative with this information, you could monitor how difficult a vehicle is to start over time, and even have HS suggest maintenance, or even replacement
"But honey! My computer TOLD me to get a new truck!" hahahh
seriously, my DS10A's should be here in a couple days, and I am going to give this a try and see how well it works. I am not really good at building things with parts, so can anyone suggest a Rat Shack part# for a relay? I did a search on their site for 12v relay and came up with dozens! It needs to be able to accept 12v & the amp's that a car battery puts out without frying it. I do have a Fluke digital Multimeter and can easily see what wire becomes hot when the ignition is turned on. I will test it out and try different events/scripts to see how accurate I can make it. I will post any finding here on the board.