Welcome to the forum! A correction - Dolby Digital is a codex that can be any number of channels, including mono. So, it isn't 5.1 by definition.
VBsafire, as for your issue, are you sure that you are watching channels with 5.1 content? Odds are that you've had at least one 5.1 program playing, but it's worth asking. Just because stations can now play 5.1 doesn't mean that all shows are in it. There are a number of stations in my area that are still stereo, but a number of them can do 5.1, too.
How do you have the cable box connected to the Yamaha? HDMI? Optical?