The Comcast HD DVR is the Motorola DVR: Dual-Tuner DVR High-Definition Set-Top DCT6412.
HDMI
I was told by Comcast that this box is the only way to get HDMI unless you use a regular HD cable box with a DVI to HDMI cable. Check out this info: DVI vs. HDMI vs. Component Video -- Which is Better?
NO SERIAL TiVo Interface
The regular HD cable box does not have a serial connection to support a TiVo.
So, if you've got TiVo and want to upgrade to HD with Comcast, it looks like you will have to use an inferior infared connection to control the HD cable box.
By removing the Serial TiVo interface from the HD Cable box and not including HDMI, they are 'encouraging' you to buy their HD DVR.
The TiVo infared controller is lame, and slow.
Motorola Dual-Tuner DVR HD DCT6412
Robert Scoble tells me that the new Comcast HD DVR uses Microsoft software. I had heard bad things about the old version of this box, but after a brief run through, I'll say it's not too bad. It has season pass subscriptions like TiVo, and the interface is intuitive. It also shows you how much space you've used up as a percentage of available space. Nice.
I haven't used it long enought to see if it has any of the freezing problems that earlier Concast DVRs had. I'll keep you posted.