You'd assume that if a CEO were asking you to sign such an agreement, they would offer some sort of incentive in return. Otherwise it seems more like an ultimatum for you to declare your loyalty.
Basically, for a contract to be valid both sides have to give something, this is referred to as "consideration".
Now, normally, with work-related contracts the consideration is "continuing to employ you", but with senior mgmt (golden parachutes, stock options, etc) I bet the waters are murky.
IANAL.