After some research and a good bit of tinkering/testing, I've settled on a design for the zero-crossing detector.

The design is based off of the one at http://www.dextrel.net/diyzerocrosser.htm
with some minor changes to fit what I had on hand.

It produces very good results:

The purpose of the zero-crossing detector is to permit phase-based control of the AC powered lamp and heatbed.  By knowing when the mains voltage is at zero, the controller can switch the AC on/off and have a variable phase drive.  If, for example, power were to be switched on for a few ms, the heat from the lamp would be very different if power came on it the trough of the AC, versus the crest.