After carefully watching in my last game, I am fairly sure the AI targets with pollution in somewhat the same manner as disease, i.e for awhile it may attack the same city over and over, though not on consecutive turns like disease does. Last game, Eridu and Nineveh seemed to get hit with pollution every 2-3 turns, this would go on for 10-12 turns or so, then stop, for one city at a time, then the game would take a break, and plop 4-5 tiles of pollution at random before picking on a city again. Strange.
Own, another thing I highly recommend is greatly reducing the time it takes to clean pollution in the editor, this is really the best timesaver, if you do not automate workers (I never do), you can lower it enough so that it is like cleaning up pollution in civ2, quick and easy. Whoever at Firaxis thought that it would be fun to have 6-8 tiles of pollution pop up every turn, and then have the player move 3-6 workers onto the tile and order each one to clean it up - well... this person should have their compilers taken away, and be forced to run around the Firaxis parking lot for the next 3 months or so, picking up litter with a tweezers.