Not as interesting as you think. Basically just a de-scrambler. Comcast is doing this to prevent the free TV service that comes with internet-only accounts. (Might actually work with phone only too, not sure)
Currently, because of how HFC/cable network works, if your drop line has service, you get channels 1-50(ish) free; because of how the technology works..
In the old days, they used to put a filter between the drop line / house feed to remove those channels if you did not order tv service. In today's digital age, its no longer viable to do this. (And many people just take the filter off after the tech leaves..)
Come March, no more free TV for HSI only customers, as-well as another equipment rental fee.
This is also useful if you have existing SD DTAs (or even STBs) connected to HDTVs (our 32" HDTV in the kitchen is such a critter - it's fed by an SD DTA currently). In other words, not exactly as you are thinking.
I had X1 installed yesterday and also asked them about these new HD Digital Adapters and she knew nothing about them as if she never heard about them before.
Comcast only trains CSRs about new products right before they become available so you probably know more about HD Digital Adapters than she does just by reading DSLReports. -- Cisco RNG 200N & SA 8300 HD DVRs-S26 Guide Links 'Guide Blog' 'Schedule' 'Info' 'Patch Thread'
In an article on the Light Reading Cable site late yesterday about Comcast now charging for formerly free DTAs (»www.lightreading.com/tv/comcasts ··· 40147394) it mentions that Comcast "is now starting to offer HD versions in select markets."
What's also interesting is that the article states that "the company is in the process of adding new features and enhancements to the millions of DTAs that are already in service. For example, SD DTAs will soon get a software upgrade that will add a guide and parental controls, a Comcast spokeswoman says."
If so, the DTA's will have a better looking guide than the full fledged boxes!
That will also pretty much satisfy most casual TV users. I know it would be totally fine for my grandparents, they don't care about DVR functionality, they don't really care about OnDemand, just watching the occasional TV program but I know they do like having a guide to help them choose the right channel.