Having a DIN sized system is a good idea if you have more than one vehicle.
For me is a matter of wasted space. In a sports car, space is already at a premium. Add in a USACi quality install and it's REALLY at a premium.
I'm having my dash customer fiberglassed to house my Xenarc and Alpine head unit. Since I'll have basically a huge chunk of free space behind the monitor it only makes sense to use it as computer space. The drives will be mounted externally, either below the dash or in the glovebox. But the solid state will be held behind the monitor.
I agree on the resale value of a carpc unless some computer nerd is buying the thing nobody isn going to know what to do when the software craps out. Even if someone I knew wanted to buy it I still wouldn't want to sell it to them because of that reason. It would have to be idiot proof and the software just isn't quite there, espeically using Microsoft stuff.
In my original plans I was going to install a slot loading DVD drive but when I was designing the case I discovered that I would have to take my LCD apart in order to slim down it's foot print so I got lazy and figured I'd stash the DVD somewhere else. It would be entirely possible to put the drive in the case if I had the time.
Finally, you are right about the newer cars not having standard size radios, at least some models. But last time I checked, not everyone on this board is driving a brand new car because a lot of us are broke college students. Besides, in about 5 years this stuff will probably come standard on new cars and we will move from hacking hardware to hacking software...
One last thing, given the popularity I bet you that VIA has at least researched building an even smaller mini-itx board. There are a lot of things you could cut off to make more room, say the extra IDE and the floppy connector, plus using a sort of port replicator system they could probably hack off another inch at least if they really felt like it.
The number one rule about computers: they always get smaller, not bigger....