So here's what I gather from this. Comcast really has 6 TV packages.
Limited basic: Local channels
Digital Economy: USA, Discovery channel, History channel, AMC
Digital Starter: ESPN, Comedy central,
P12 Mountain channel,
Digital Preferred: NFL network, ESPNU, BYUtv,
I think you have to order the "sports entertainment package" to actually get the PAC12 national channel.
https://www.comcast.com/Clu/ChannelLineUp
I've got digital starter and I do get the pac12 regional channel. One thing that I think sucks is I have a digital adapter for the upstairs TV and it does NOT get the PAC12 channel. I was told that's the way it is from the Comcast chick today.
Another note: This is early. So far, the national channel is showing exactly the same thing that's being played on the mountain channel. All day, every day. I'm sure that will change as we get more programming material. Just a suggestion that you might not have to rush to get the national channel around here.
Hope that helps guys.