It seems that there are two numbering systems in humans – a general sense of numbers for some quick and dirty estimates and some more genuine computation skills of showing the result of (2327)^2. At least the first capacity seems to be inborn (and an important survival skill). According a recent SZ article (29th Sept 2008) a host of new studies now show that
the two number systems, the bestial and celestial, may be profoundly related, an insight with potentially broad implications for math education
and as I believe – for science in general as most science fields are being dominated by the Fermi problem. A new and exciting study published on Sept 7, in Nature shows
that individual differences in achievement in school mathematics are related to individual differences in the acuity of an evolutionarily ancient, unlearned approximate number sense.
So we will probably have genetic tests for that as well as for sensing the perfect pitch. Oh no! More serious work on numerical representation may be found in Dehaene, S. The number sense. New York: Oxford University Press, 1997; Cambridge (UK): Penguin press, 1997. ISBN 0-19-511004-8.