After poking around a bit more, I've gathered a little more info on some of this.
DVI can be used to transmit a number of different kinds of video signals. They seem to break down like this:
VESA 24-bit Digital RGB (used for computer displays)
EIA-861 24-bit Digital RGB (HDTV)
BT 656/601 Digital Video
Analog RGB (also for computers, and the same as VGA uses)
(Analog RGB requires DVI-I, but the others only appear to require DVI-D.)
The relevant formats here are VESA Digital RGB, and the EIA-861 Digital RGB. VESA Digital RGB is what's typically used for DVI computer monitors, while EIA-861 Digital RGB is typically used for HDTV.
The HDCP-DVI ports on current TVs should be designed for EIA-861 Digital RGB. (The specifications for each TV should indicate whether the port is EIA-861 or VESA compliant.)
The EIA-861 specifications include resolutions and timings specifically for ATSC HDTV formats such as 480i, 480p, 720p, 1080i. Aside from the resolutions and timings though, it's difficult to see much difference between the VESA Digital RGB and EIA-861 Digital RGB. Both appear in essence to be 24-bit Digital RGB. (It's unclear to me though if this RGB data has to be converted to YUV before being displayed on a typical HDTV, since the CRT itself uses RGB. One would think that this should not be necessary on a true digital TV.)
Since EIA-861 Digital RGB is designed for HDTV broadcasts, video transmitted in this standard may typically be pre-filtered for NTSC/ATSC compliance. Analog NTSC YUV video is limited to 220 levels of luminance. Values beyond this are not considered legal for broadcast. To insure NTSC compliance, the palette of 24-bit RGB video, which normally has 256 levels, is usually clipped below 16 and above 235 in video intended for broadcast.
The question is: could RGB values outside this potentially damage an HDTV that is only intended for NTSC/ATSC compliant EIA-861 RGB? The couple of video technicians I've spoken to say they've never heard of illegal RGB values damaging an NTSC monitor, except in some much older video equipment where different voltages were also involved. Their experience with the new DVI inputs may be limited though. And this would not be as much of an issue with other kinds of inputs such as Composite, S-video and YPbPr, because these all use an analog YUV color space, rather than RGB.
However, I also spoke to a manufacturer of a high-end video scaler which includes both VESA Digital RGB and EIA-861 Digital RGB options for their DVI output. And they told me that they had no trouble using the VESA computer mode to go into the EIA-861 HDCP-DVI port on some Sony HDTVs they tested.
Added 5/3/03: In addition, though I haven't see the EIA-861 specs in the flesh, so far I have seen nothing to indicate that it contains any particular palette requirements on top of it's HD resolutions and timings. It's usually just referred to as a "24-bit Digital RGB" format.
In the absence of any hard facts from the TV mfrs themselves, I guess my conclusion from all this is that the chance of harming a TV by attaching a computer to the HDCP-DVI port is probably relatively low, particularly if a restricted RGB palette is used. I think alot of the admonitions by mfrs and salespeople against doing this are more related to burn-in, ignorance and towing the company line than the capabilities of the DVI port to handle computer signals.
However, if a YPbPr computer input looks acceptable, and you plan to connect other STBs to the DVI port, then there's probably no point in taking even a small risk of damaging it.
And there may be variations in how this would work, depending on the TV and videocards involved.