Sunny will definitely know more about this than I, but in addition to geographic maldistribution, there is also a differential demand for specialties in different areas and at different times.
I know there are some studies out there saying there is a shortage of general practioners and primary care physicians, and that the HMO system has fostered a growth in these more fundamental disciplines. Some specialties are supposed to bloom like cardiology, oncology, geriatrics, as the population gets older. But for real info, I think there are some actual studies that take a look at this, and Im sure some of the wiser med students will know where to point ya.