This would hardly solve the problem. By the time they would have you decrypt, they would have made a disk image of the drive in question. When you enter the killswitch, they could simply restore, and request that you try again, not to mention that that is then suddenly better evidence against you.

The best solution seems that it may be nested encrypted partitions. ala, the trucrypt solution: http://www.truecrypt.org/docs/?s=hidden-volume