Here is an article on the preliminary results from Mars Odyssey's radiation environment experiment.

The image shows pretty much what one would expect, I suppose -- that cosmic ray intensity is higher at higer elevations like the Tharsis volcanoes and lower at lower elevations such as Hellas.

The interesting part is that the entire range (10-20rem annualized) is lower than that for the ISS (20-40rem annualized).

Yet another way in which planetary settlements beat orbiting colonies.