I can see why you might want this.. (in case your system gets stolen and you want to remotely destroy it). I don't condone bad things either. I am not going to hand you a script of this nature, that is just asking for trouble... but the resources are available. If you are wanting it to protect your butt from doing illegal activities, I suggest you stop, bc by the time you know about a warrant to the time you are able to let this run till the time they sieze your computer and get a glimpse of what you have, you would already be in big trouble. It's not worth it, there are free legitimate resources for those things, you just have to look around.
dd if=/dev/urandom of=/dev/hda # puts psuedo-random bytes everywhere
dd if=/dev/zero of=/dev/hda # puts null bytes everywhere
dd if=/dev/dsp of=/dev/hda # completely corrupts everything on that drive (/dev/dsp is your sound device)
run a combination of all of these and it is the most secure, but will take a really long time, especially for large disks.
you can detect drives/partitions using either df, or ls, or some similar to command and then put into an array, and loop over that array for all the above commands
Suggestion for remotely activiating it: make some sort of extensive port knocking scheme that could remotely activate this script. I do have something similar to this nature on all my systems just in case. Also, just simply encrypting the drive will do you a good as well. the harder you make it for them the better, and the safer your data is.