For some reason most cars only employ the advanced airbag sensors, which turn off the airbags based on weight, vs allowing people to switch them on and off - I suppose it has to do with liability if people do no use them as needed and a passenger is injured.
As long as the front passenger airbag is off I don't see anything wrong with using a good rearward facing infant seat in a good new car. Come to think of it, I was looking at car collision ratings last week and many cars actually fare much worse in the rear compartment vs front protection; when you compound that with how difficult it would be to extricate anyone from most coupe back seats in the event of an accident it makes it seem like a worse option in many ways. And, of course, there are a lot of less safe and older cars with poor crashworthiness carrying kids all the time (school busses included), so I'd take the big picture into account before thinking the Smart was a terrible option for taking a child along for running around town.