I just saw this newspaper Q&A from almost 10 years ago so apparently the lack of a rear middle seat head restraint has bothered some prospective Chevy customers for a long time!
"I've been looking at many new five-passenger vehicles, specifically the Chevy HHR. One thing that bothers me is that there is no head restraint for the middle passenger in the back row. I can understand the visibility concerns associated with locating a head restraint in that position, but because I have three kids, I'm more concerned about the safety of the person sitting in that seat." from: October 2008 http://archive.boston.com/cars/researchandadvice/2008/10/09/the_car_doctor/
"I've been looking at many new five-passenger vehicles, specifically the Chevy HHR. One thing that bothers me is that there is no head restraint for the middle passenger in the back row. I can understand the visibility concerns associated with locating a head restraint in that position, but because I have three kids, I'm more concerned about the safety of the person sitting in that seat." from: October 2008 http://archive.boston.com/cars/researchandadvice/2008/10/09/the_car_doctor/