The Real Scoop:
I'm a Home Theater Installer for Circuit City, so I deal with these boxes on a daily basis.
Certain TVs don't require the source to use HDCP. Among these are the Hitachis and JVCs. Newer Sony sets (at least as far back as when they started branding their LCD engines as Bravias instead of Wegas) are a no-go, straight through to the tv and through receivers.
Comcast does not use HDCP. Why? Well I've gotten some inside info from Monster Cable, whose been trying to solve this issue as much as anyone. They are getting record returns on HDMI cables because of HDCP issues. People have been blaming the cables as being defective, but thats clearly not the case.
The reason is that Silicon Image, the company that developed HDMI, DVI, and HDCP handshaking, requires companies that use HDMI to pay royalties in order to have HDCP handshaking. Comcast has decided that they are too cheap to pay it.