" perhaps the 'dope-slap' (anti-whiplash) mechanism has been triggered in the one you drove? Are the rests removable and just placed in backwards?"
That could well be. Peter says he only tried one 2006 the day he was looking.
Last year we were looking at 2005's, and I had a similar problem (he didn't) in the Legacy. We did try reversing the headrest, switching out the backseat headrest. Great minds must think alike.