Thank you for your replies. After talking to a few people about this, I think I've decided against using an Arduino. Spending money seems to be the safest way to go here.
We will probably end up using a DMX interface (through ARTNET and interacting with Quartz), which will go to several DMX Dimmers and from there to the LEDs. I realize this is an Arduino forum but does anybody have any tips or warnings to give me?