I just completed The Einstein Syndrome by Thomas Sowell. It concerns exceptionally bright small childred who are also exceptionally late at beginning to talk. He (and others) have identified a few hundred of these kids nationwide. They hypothesize (for whatever reason) the development of the brains in these kids disfavors the speach area for the first 3-4 years, in favor of other cognitive regions.
We're pretty sure our son Ben is one of these kids. He has just turned five, and is extremely smart (he was able to add and subtract in his head before his third birthday, and he is now starting to read words - although we haven't taught him to read, and he admits to not being quite sure how he is doing it). He also didn't begin to talk until he was nearly four. Tests showed that he went from speaking at the level of a 12 month old to speaking at the level of a 7 year old in about 15 months.