Blogger "scaryice" of Climbing the Ladder has compiled plus/minus ratings for the 2009 MLS season, with raw and adjusted data that give some results that might be surprising to people. I've thought of plus/minus ratings at various times in the past, but I'm not so sure that a plus/minus rating translates well to soccer. I admit that I haven't thought through my initial impression, so I am willing to revise it if proven incorrect. It does gives me something to think about during the flight tomorrow, and during one of the paper presentations that I'll see on Saturday.
At any rate, the linked article is a good starting point on a potentially interesting subject.