On Demand & CATV set-top boxes never transmitted data over your HSI connection.
They can and do in some cases. Most Motorola set tops made in the last 5-7 years have a built-in cable modem. It can be used for streaming IP VOD, multicast video, and most often used for the set top to transmit data (ie VOD control) back to your cable provider. It's not all that commonly used but it definitely blurs the lines. Simply transporting traffic via DOCSIS doesn't make it Internet traffic. For example Comcast's digital voice service has never counted against caps either.
I would be doubtful that any set-top boxes built-in modem uses the same channels or network as the customer's HSI. I don't think the issue is that Comcast might do two-way digital communication, it's more an issue on how Comcast classifies & regulates it's services.
CDV is an interesting thing & is blurring the lines somewhat. However, I think the fact that they require internally separate proprietary hardware that accesses their private VOIP network w/ QoS to avoid interfering with the customer's HSI, makes it less shifty vs Streampix which explicitly uses the customer's HSI because it must if it's to work on a phone or Xbox.
They do. Cable modems are provisioned with multiple service flows much in the same way cable digital phone service works. I've seen this setup in a lab being run from one CMTS. The modem has a service flow for Internet (say 50Mbit/sec) and another for the eSTB (say 30Mbit/sec) which can handle the IP VOD and IP multi-cast video. It is possible to have multiple CMTSs for this however both Cisco and Motorola are investing in VDOC (video-over-DOCSIS) scale solutions that allow you to put all your eggs into one huge basket. One cable modem / gateway will ultimately be feeding every cable service in your home and some portion of it will be provisioned for plain ole' Internet traffic while the rest of it will be allocated for MSO services. From what I can tell this is not how Comcast is delivering IP VOD to the Xbox today. I would expect them to make the transition fairly quickly though. This stuff has been up and running in labs for the last 5+ years. MSOs are just very slow to adopt new technology.
I think it's technically impossible for Comcast to get video to the Xbox w/o using the IP connection provided by the customer's HSI service. I would say, so long as it's using the same IP the customer would use to get on the Internet with, that it is using HSI and should be subject to whatever limitations they place on HSI.
They can and do in some cases. Most Motorola set tops made in the last 5-7 years have a built-in cable modem. It can be used for streaming IP VOD, multicast video, and most often used for the set top to transmit data (ie VOD control) back to your cable provider. It's not all that commonly used but it definitely blurs the lines. Simply transporting traffic via DOCSIS doesn't make it Internet traffic. For example Comcast's digital voice service has never counted against caps either.