I received a $200 check yesterday as well. I genuinely have no idea why--I don't recall missing out on any bonuses. @Eujeanie--the check looks completely legit--drawn off JPMorgan Chase Bank. There aren't any "weasel words" anywhere that would lead anyone to think that cashing it enters one into a contract of any sort. In fact, the letter states "You don't need to take any further action before using this check."
Weird--but I'm inclined to take it at face value (which is $200!)