I believe the N900 back cover has a magnet on it that allows for the device OS to know when the back cover is on or off, right?

Could you incorporate a feature that would send out an SOS when the back cover is removed(assuming the device is on, of course - and works with the security lock?), using a brute force, no holds barred method within x seconds, where x can be set by the user?

I think this sort of feature could be very useful, and we all know the battery has to be pulled first before the SIM can. So if the device is on and locked with a security code, a thief may want to pull the battery so the device can't be tracked, if they are smart enough to realize that possibility. If your device is unlocked, they are more likely to play with it first hopefully, but a locked device has no immediate return value for a thief and they will may want to power it down.

I reason there would be about four to six seconds once the cover is removed until the battery is pulled, if a thief is stationary and taking care not to damage the device, not rushing basically.

First they look for how the back cover is removed, find the notch and give it that tentative yank we all did when we first got our device. IF the OS registers the cover off here, with it slightly ajar, I would say the time til battery is pulled could be five to seven seconds. The thief finally fully removes the cover and puts it down. Not drops, not throws in a hurry, but puts it down.

Second, the thief takes a moment or two locating the battery and the proper end to pull it from. If they have long enough nails, they can do it easily with just their fingers. If not, do they have a tool handy or do they have to fumble for the stylus(if they know about it) or grab the cover to use one of its corners?

My only concern is I am not sure if the device or your app can notice the cover removal when it is locked?? If they both can, it's probably easier to implement this than trying to figure out exactly what to send in basically < 5 seconds...