Yes, the AppleTV has an optical output. It also has video outputs (component and HDMI, plus maybe composite) so you have an on-screen interface for controlling it, and a hard drive for storing some content on it locally. Check it out for specifics.

By the way, I looked - both the iMac and Mac Mini have optical outputs, but they are the 1.8" mini-jack style also found on the Airport Express, so you can't use a standard optical cable. The AppleTV actually has a standard optical connection.