Spencer the Katt gives brief mention to a forthcoming device that, like the Lojak car system, gives location information on a lost or stolen laptop. If you accept the premise that a thing that is not obvious but not entirely invisible is a useful anti-theft technology, couldn't equivalent functionality be done in software? Essentially, ping a server with your id, and if you don't get an okay, disable the system (yeah, yeah, work in a semi-disconnected model, user-definable lockdown options, virus-writer techniques to mask the driver's "signature," etc.). Of course, short of a BIOS implementation, it would be paved over by a formatting, but you've achieved the primary goal (casual access to your information). Maybe the lockdown could lead to a "No questions asked reward" screen -- giving the theft / janitor who found the thing an incentive to return the hardware in the locked down (but recoverable) state.
I dunno'. Are laptop thieves actively interested in purloining information? If so, putting a software-based obstruction in their way is laughable.