re: airbags vs. Viking
I've read that the early plan for Mars Science Laboratory, which would land in the last half of 2010, has a delivery platform that would use retro rockets for the powered descent. But the twist is that the "Sky Crane" would hover just above the surface and lower the rover on a bridle before flying off.
http://centauri.larc.nasa.gov/msl/PIP-Drft_031121secure31.pdf