Your unit is an "SD" unit, meaning that it is standard definition TV, not high defintion TV (HD unit).
I have not checked for quite a while, but there used to be only 2 or 3 PPV channels that actually offered DD 5.1 audio, and the program guide showed so. HBO and Showtime used to also on their main channels.
The Dolby Digital 5.1 sound is the same as your DVDs typically provide, at least on the newer movies.
For non-DD 5.1 programs, the optical output provides 2-channel stereo, sometimes with matrixed Dolby Pro Logic surround, just like you get from the L+R analog audio outputs of your DirecTiVo.
a total DUH moment for me occured when i got my TIVO back in november -- i hooked up the optical output from it to my receiver and actually noticed for the first time that my DD light on my receiver actually lit up. where the hell have i been all this time??
but i have noticed better audio separation even on non DD channels since i hooked up the optical. i'm also running optical from my Panasonic DVD player/recorder as well, but still run the analog outs from my Pioneer DVD player for DVD-A/SACD.