I had a similar experience with CSRs earlier this week when trying to order my second box. On two occasions, I requested 6200 and they both told me that they were not available. But they both told me that the Comcast guy would come with the latest box available so I gave them the benefit of the doubt.
When the Comcast guy arrived today, I asked him for a 6200 because I wanted the DVI output. At first, the guy told me that he had only 5100s but that he would go take a look. He came back with a 6200 in his hand, saying that he thought they were all 5100, but that he had actually only 6200 in his truck! To his defense, he showed me the box, and I had to agree with him that besides the 2 Firewire ports and the label, the boxes look almost exactly the same.
Since he had plenty of them, he even offered to exchange my 5100 for a new 6200. So I actually have 2 of them here now.
I also did got the new "On Demand"-ready remotes, even though the service isn't available here yet.
So, my advice to you is to not worry too much about what the CSR tells you. Try to get them to understand that you need a DVI output on your cable box and you would like to replace your current one. Whether they think you'll get another 5100 (with DVI) or a 6200 is irrelevent, chances are that you'll get a 6200 at this point.
My $0.02 for what they're worth.