I looked at this a bit more just checking fuses and listening to how the system was performing. I noticed that there was a noticeable change to how the airflow sounded when I would move the temperature knob from hot to cold, so I figured that it might not be the blend door.
A few days ago, I got a check engine light. Then a day later, I got a overheat warning light. I quickly slowed down to take the next exit, and the overheat light went off. I got to work and checked the coolant reservoir and sure enough it was empty. I had someone give me a lift to the nearest auto parts store which in this case is a big box home center (hey, they carry premix Dexcool!) and filled it up. And yes, my heater works now.
I checked the car w/ my trusty old (really old) OBD scanner and got:
P0117: Engine Coolant Temperature (ECT) Sensor Circuit Low Input
P0128: Coolant Thermostat (Coolant Temperature Below Thermostat Regulating Temperature)
So perhaps I have a slow leak somewhere like in my water pump because I don't see any leaks even right after driving it.
I'll just keep an eye on my coolant level for now.