It doesn't really mean anything, most of the time. Others with more experience in this area can correct me, but I'm pretty sure it isn't very meaningful.
FP doesn't really have the well-developed system of fellowships that IM has. Doing an FP fellowship doesn't garner you the kind of income and hospital privledges you can expect from a IM fellowship like cards or GI or whatever. Often you spend extra time in the fellowship, and then go right back to what you were doing...but now you're better at one segment of FP medicine, according to your CV. I know some hospitals won't honor a CAQ for C-section privledges - they don't see it in the same light as the certificate you get for completing a IM fellowship. They are more likely to simply honor the number of sections you've done rather than the fact that you have a CAQ. After an IM fellowship, you're a specialist. After a FP fellowship, you're still a generalist (in some people's eyes).
That said, the CAQ is something you can put on your wall for the young Michelle Qwans to see when they venture into your office. Organizations that may hire you as their sports doc will be receptive to it, and just maybe it will give you an edge over a similarly qualified doc w/o the certificate. Emphasis, however, on the maybe.
So, the decision to do an FP fellowship should probably focus on the kind of experience you can get rather than whether or not you'll get a CAQ. In the end, the CAQ won't really help you much.