Here is the list of FCC TV Allocations
http://edocket.access.gpo.gov/cfr_2004/octqtr/47cfr73.606.htm
Of course with DTV it's all on hold and will be adjusted
As for markets, when they were established no one had any idea there would be anymore than 2 networks. So in the East and Midwest were the populations were denser markets were simply established. As the West grew, markets were larger because at the time they had less people. For instance in 1950 San Jose had only 95,000 people. Today it is hovering under 950,000 people. San Jose would've been its own market today but it's too late for that now.
So markets are arbitrary, some like DC and Providence are political. DC and Baltimore could easily have been one. Perhaps one could say for Boston and Providence. (Locating the transmitters halfway) But because they are capitals there is politcal factors to consider.