The digital out only passes 5.1 audio from the tuner, that is correct.
If you're sending other devices to a receiver, they would need to be connected either through the receiver (HDMI pass-through) or with a separate optical cable.
The digital audio out on a Samsung UN46C6800 is a big disappointment! This TV only outputs 5.1 CH sound when receiving a DTV signal on ANT-IN. Devices connected to HDMI inputs only pass 2 CH to the Digital Audio Out.
Am I missing something here? All the newer devices use HDMI for signals. This TV has 4 HDMI inputs and none of these inputs pass the 5.1 CH audio to the Digital Audio Out (optical)?