If the signal is going down HDMI it is digital. It may be your cable box that is taking the standard digital and analog channels and converting them to HDMI so that you can switch them on the 970 and then send them to the Plasma. But since the HDMI cable itself is a digital signal it is more immune to interference than analog.

A purist would say don't use the same conduit. My personal experience with this sort of stuff (although I haven't done much HDMI I have a bit of digital experience) is that as long as you have appropriate quality cables you will be fine. I'm not saying you need insanely expensive cables, just ones that have good size conductors and proper shielding. Especially with just a 5' run.

Like gong said your best bet is to bundle them together before you put them in the wall to ensure you will have no issues.

Also, if you have analog cable, and you don't know if your cable box/satellite box/whatever box will pass the analog channels via the HDMI you might want to check on it just to be sure. To get them on the HDMI they have to be converted to digital at some point, and I'm very sure the 970 can't do that for you.