[StoneTable]

You could use a laptop with some kind of sudden motion sensor and use that to lock the machine if the machine is moved.

[tacoman]

Thinkpads have this sensor that is meant to protect the hard drive from sudden shock. I bet it could be used for something like that.

http://www.thinkwiki.org/wiki/Active_Protection_System

[lsllc]

A bit old, but Amit Singh did some interesting work with the sudden motion sensor on the Mac Powerbooks:

http://osxbook.com/book/bonus/chapter10/sms/