I came up with a theory last year as to why a lot of people (especially black) don't hear this chirp; it has to do with Basketball:
The reason is because copious amounts of playing and watching basketball, makes the brain ignore high-pitched noises. The chirp of a smoke detector sounds the same as sneakers squeaking on the gym floor during basketball.
Since this is a constant sound, you tend to tune it out. Unfortunately, you also tune out other noises, such as a smoke detector chirp.