Arduino Christmas light project with dimmer
I can think of a couple of ways to approach this:


Use a DMX dimmer, something like this 4-channel dimmer, or
Use an household X10 dimmer.


DMX is used for stage lighting and there is some information on controlling DMX devices with an Arduino in the Playground where there is also some information on the hardware interface and shields, it looks pretty simple. DMX is a differential RS485 interface.

X10 is used for home automation, the signaling is done over the AC line. There is some information on using X10 and an Arduino library in the Tutorials.

Both seem like viable approaches, if you're working in a theater it is likely that they might already have DMX equipment and it would be as simple as hooking up to the gear in the house. Then you'd only need to add a shield or build the interface hardware.