A computer shop by me is selling them for 50 bucks - they ordered a bunch, and nothing sold.
I might pick one up to take appart.
they are designed to supply power to the motherboard when the ac fails- Im not sure if they hook into the at(x) power feed... or if they send power through the bus.... ether way, it has dc-dc circuitry in it.
It could be modified for car use, but the thing is - they were designed to run for a short period of time, you would need to make some changes, add a fan, some other stuff to use it as a permanent replacement. Ill see what I can come up with.