I posted some info on another forum site about this topic, but I wanted to post some here so it can be found in future Google searches.
Of course, the information I am sharing is based on my own experiences, sadly without the cooperation of Motorola-- as they absolutely refused to talk to me about any of my findings, etc.
The Motorola DCP-501 shares a digital cable box model, similar to the DCT2000 series boxes out there. One major difference in the DCP-501 is that you, the end user/owner, can make changes to settings (audio settings: stereo mode, surround settings, etc., VCR record settings, etc.) that are stored in a spot in the flash memory. This is where things go wrong with the DCP-501 and the new firmware Comcast is pushing out (in my area, it is version 8.09).. The firmware needs more memory and tries to take the additional flash for it's business. The result: not all of the flash memory gets written. This leaves the box in a non-usable state (yes, it turns-on, but nothing works properly).
Comcast 'could' make a working firmware pushout for these boxes if they desired. The magic combination is to keep the base firmware at 7.59 and load the A28 TVguide update, along with the VOD update and interactive plugin. It could be called a success, but uses-up a lot of memory and leaves you with little "future" guide listings. Unclear if it supports SDV. The code appears to be there, but not used in this area yet.
Not much chance you will get them to make this firmware for you due to the fact that the agreed solution was to formerly have Motorola drop all support for the DCP-501 box.
One thing I may look into is to upgrade the actual flash memory chip to something more reasonable for today's firmware. This is not an easy task, and the odds are against me from the start (I dont have a solder flow station, etc.)
Before I waste all the efforts, I am looking into a legit source to get some Motorola DCX series units that are legal to purchase and have a Comcast-provided cablecard paired-up for use to the channels I subscribe to. Possibly I will get a cablecard rental fee to deal with. I have not heard of anyone getting out of multiple cablecard rental charges.
If someone has a DCP-501 that is not working (black screen, strange display when powered-up, etc) that is registered on Comcast, send me a msg.