Well, after several weeks of reading, looking at others' schematics, troubleshooting and talking to the folks at my local electronics place, I believe I've finished the circuit I've been trying to develop.
It's designed to detect the zero crossing of an AC line and control the dimming of several incandescent bulbs, as well flash the bulbs in various patterns. (My schematic shows only one load control, but there are others identical to it connected to different digital pins on the ATMEGA.)
Pots control the dimming level and the speed at which the lamps chase. A momentary switch cycles through various patterns. I've also moved off of the Arduino and made the circuit standalone.
Everything appears to work, but if the more experienced folks here wouldn't mind looking at my schematic for potential problems, I'd certainly appreciate it!