I knew I had seen something about head restraints last year knocking the fortwo and others out of Top Safety Pick and just found it. I would presume the same reasoning follows for the 2010 models since they are unchanged. Of course, I don't understand what the deficiency is?
2009 Top Safety Picks by IIHS
"Twenty-six models fall short of earning TOP SAFETY PICK because of inadequate head restraint designs. The Smart Fortwo, the only microcar in the US market, missed because of its head restraints. The same goes for Toyota’s hybrid Prius, which performed well in the Institute’s front and side crash tests but came up short for rear crash protection."