Have you considered the fact that the thief is not going to be able to charge the tablet unless you happen to have your charger chained to your tablet at all times. This has certain implications:

1) The window of opportunity for any recover attempt is rather small. A day or two maybe. It needs to be pretty aggressive in recognizing that the tablet is stolen and collecting the necessary data.
2) The thief is most likely stealing the tablet to pawn it off somewhere else or at least will do so as soon as it is dead. Also they likely have no clue as to how to operate it. It is fairly intuitive but not *that* intuitive. The likelihood of a thief to research how to operate it and which charger to buy and then go out and find one is unlikely.
3) Given 2), your most likely utility of this app is recovery of the tablet from some unsuspecting mark who it was sold to off the back of a truck and not in capturing of the thief.

Keep these things in mind when designing your triggering frequency and what data it collects.

The GPS data of the complete track from theft detection to recovery might give invaluable info to the police in finding the thief even if it was fenced to someone else.

Capturing a photo of the thief might require some social-engineering to get them to activate camera capture at the right time (have it ring like a phone or something and blink some acknowledgment message on the screen that says "Press here to answer/hang up" :]).

Use the ambient light sensor on the N810 (if you have one) to detect when it has good conditions to take some pictures or handling of the tablet. There is no point in transferring a bunch of black pictures of the inside of someones pocket.

Even if wifi is not available, taking pictures and GPS data in a "store and forward" mode when wifi is around will give more opportunities for good data capture.

Just a few thoughts that may have already been mentioned.