Updated October 20, 2014
Software
If you’re permanently installing the tablet in your car, or planning to leave it in the car at all, you obviously have a greater-than-zero risk of getting your tablet stolen. In the hands of a thief, your unlocked tablet is a gateway to your Gmail and an easy road to stealing your identity. To protect against this, use Tasker to set up the following, at minimum:
Task name: "Lock on Power Loss"
Event: Power Source Any, Invert (translation: loss of charger connection)
Task: System Lock, Disable Wifi, Disable bluetooth
This will lock your tablet and turn off the screen when you turn the car off. Note: Tasker will re-enable Wifi and bluetooth when the event stops (when the power to the tablet is restored) so you don’t need to set up a second task to re-enable them.
Next, set up a password of some kind on your lock screen. I prefer a PIN because it’s quicker than a password and doesn’t leave a distinct smudge like the pattern lock which the thief can use to deduce your password pattern. You would still be wise to use a cloth or similar to wipe off the PIN-pad area of the screen on a regular basis so that you aren’t leaving clues. 2014-10-20: There is now an option available in Cyanogenmod where you can have the tablet automatically unlock after the correct PIN is entered, without you having to tap the enter key. Saves a tiny bit of time/effort.
Doing the above will not prevent or deter theft, but at the very least, the thief will have to wipe the tablet completely to be able to use it, destroying all of your personal information in the process.
Hardware
I created a façade which obscures the tablet and makes it look like I still have the original stock 6-CD changing, cassette-playing head unit installed. To do this, I removed the front panel from the stock head unit and cut away material on its sides to allow it to fit into place on top of the tablet. It doesn’t look exactly the same as a normal 2002 Subaru stock head unit, but it fits well and it’s close enough that any thief peeking through the window will clearly see a cassette slot and (hopefully) move on to the next car rather than trying to break into mine.
2014-10-20: I also bought and have been using a steering wheel club as an added deterrent. This is a big, obvious, highly visible indicator that the car owner has taken steps to prevent theft, and if a thief is browsing cars trying to decide which one to break into, the one with a club and a cassette player sure doesn’t look like an appealing target. I use this one, and I like it a lot — it’s bright red, easy to stash in my passenger seat footwell within reach, easy to lock/unlock, and feels nice and sturdy.