I purchased a brand new DCP-501 on Ebay. It arrived last week and I appreciate all the great information on this thread. Happily I wanted to report I had a very good experience with Comcast. There were several calls involved, but some of that was because I was not home when they called me. The first time they provisioned the box, all I got were what I believe was the clear QAM channels or some sort of very basic package. I went into the diagnostics mode of the DCP-501 and recorded every screen in SageTV and sent them a word document of all the screens and sent them an email last night. Came home tonight and they had replied to my email. They could see nothing wrong in the config, but sent a "cold init" to the box instead of just the "init" that was sent earlier. Bingo fired up SageTV, box is working perfectly, gets all my SD digital channels including the premium channels, guide works, on demand works, picture is great. So for me it seems the magic word is "cold init" after it is provisioned.
I cannot say enough good things about the support I got from Comcast. Talked with several people there, they were all extremely helpful. 2 of them gave me their direct lines and emails, no one passed the buck.