A couple days later I called them about a DVR box that had given up the ghost (individual segments of the front clock feebly flickering green). While I was on the phone I asked about the service, and was told "oh yes, you're eligible; it'll be $20/month over your current RoadRunner Turbo and gives you 5mbit up as well". When I said that sounded good, but not $240/year good, he told me about an "Extreme" pricing for $10 less which was 30/5. $10/month extra for 5 up sounded awfully good, so I scheduled that along with the DVR replacement.
The service tech showed up 15 minutes early (I showed up 10 minutes early for the appointment, but he waiting patiently!). We swapped the cable modem for a Ubee cable router, with 4 ports and wireless N. He asked me if I was a techie, gave me the password, and stepped back while I made sure it worked. It did, gave about the speeds promised, and he was on his way.
After I reconfigured everything to use the Ubee instead of my D-Link, the only problem I've had has been with the bind DNS server I run locally. Responses were just really, really slow. Often timed out in fact. I tried sticking my DNS box in the Ubee's DMZ, which didn't help at all. Using the Ubee as a DNS forwarder worked just fine, whether the forwarding was from bind or from a client.
Finally I decided this might have something to do with the source port. This was part intuition, part wondering if the router was trying to protect me, and part guess. I fixed the source port to a randomish port number in the bind config, and all was happy. In case anyone else has this happen, make sure you're not binding your source port to port 53. I don't really know why I was, but once I stopped everythihng worked fine.
Partial fail for the Ubee -- the DMZ should be the DMZ and not filtered at all. Overall a win for TWC, who is giving me 20mbit down and 5mbit up over their network pretty reliably. Netflix and Amazon video streams start noticeably faster; I think they look better too, though that's likely the techie equivalent of beer goggles.