If you're under warranty still: dealership.
If you're out of warranty: Pull out the glove box assembly (as you would to access the cabin filter), then check the blower assembly (right below the cabin filter slot) and damper system (behind the stereo) for jams or cracks. Perhaps some of the dampers aren't actuating properly.
With the car warmed up, turn the heat on full blast and cycle through all of the vent modes. Take stock of which vents blow cold for each mode. Don't forget the ducts under the front seats (at least my bugeye has ducts there, not sure about the 3G cars).
EDIT: Please don't start more than one topic for issues. Mods, please merginate this in the other thread in the Maintenance/Repairs forum.