"Researchers at the medical university Karolinska Institutet and Umeå University have now demonstrated a correlation between general intelligence and the ability to tap out a simple regular rhythm. They stress that the task subjects performed had nothing to do with any musical rhythmic sense but simply measured the capacity for rhythmic accuracy. Those who scored highest on intelligence tests also had least variation in the regular rhythm they tapped out in the experiment."
Here is another quote about the study that seems to go against some of what we have been told about the relation between brain structure and intelligence,
"They also demonstrated a correlation between high intelligence, a good ability to keep time, and a high volume of white matter in the parts of the brain's frontal lobes involved in problem solving, planning and managing time.
"All in all, this suggests that a factor of what we call intelligence has a biological basis in the number of nerve fibres in the prefrontal lobe and the stability of neuronal activity that this provides."
The study appears in 'Intelligence and variability in a simple timing task share neural substrates in the prefrontal white matter', Fredrik Ullén, Lea Forsman, Örjan Blom, Anke Karabanov and Guy Madison, The Journal of Neuroscience, 16 April 2008. But I got my information from an article at Science Daily.