I might be hesitant of putting it on the vent itself. Having the cold air blowing on it, chilling the interior, and taking it off to walk around in a warm pocket might cause condensation issues. That also might lead to problems with moisture on the inside. That can't be too great for circuits.
That being said, I think I am going to try one of the ones that fits in the CD player. There's a nice phone-sized gap in between the vents that would prevent the cold air blowing directly on the back of the phone. If my phone dies in a year I'll let you all know.