This is a Carrier Comfort 80 58DLX155.

Turned out the cause was a faulty earthquake valve ( http://en.wikipedia.org/wiki/Earthqua... ). I live in California and the earthquake valve on my side of the meter had partially tripped, restricting but not stopping the flow of gas. I had PG&E come out and take a look. After they tested pressure at the meter (which turned out fine) they reset the earthquake valve and everything works again.

Here's what an earthquake valve looks like : http://www.earthquakevalvespecialists...