[schoen]

There also seem to be a number of U.S. cases enforcing unilateral reward offers as contracts; a theme seems to be that they intended to get other people to do work and those people reasonably relied on the offer and did the requested work (most often going out and finding a missing person or item).

[icebraining]

I believe the doctrine is called Promissory Estoppel: http://en.wikipedia.org/wiki/Estoppel#Promissory_estoppel

[schoen]

Yeah, it looks like that's the theory.