Just though I'd give an update. Just yesterday I started getting steam coming out of the vents, window coated with slime, smell coolant, and now have it dripping onto the floor. I'm pretty convinced it is the heater core now. I found a guy who would replace it for $300, does that sound reasonable? He says it is about a 3.5 to 4 hour job.

FYI, thermostat and radiator were replaced last winter after the radiator cracked in 20 below weather (not fun). Probably because I took it in for an oil change at a Wal-Mart in California and I suspect that they added water to the radiator instead of topping it off with coolant. Needless to say I now do all of my own oil changes.