http://www.mbusa.com/brand/container.jsp?/care/clientcare/roadsideassistance/index.jsp&menu=2_3
not very informative, there's also a brochure at the dealer with some old guy in a 300SL Roadster driving down the road and "ROADSIDE ASSISTANCE" in big letters printed on it. It gave a lot more detail.
From what I know they'll change a tire, give you some gas, or jump your car for free. Everything else has a small fee.