Considering HDTV Copy-Protection Systems
Film and television studios are worried about people copying their programs and distributing them to others. HDTV (and DTV in general) is worrisome to the studios because people might be able to make "perfect" digital copies. (Nondigital copies get worse when they're recopied, so they aren't such a threat.)
These content owners have lobbied the government and manufacturers to include copy-protection systems in HDTV devices, such as tuners, set-top boxes, satellite receivers, and HDTVs themselves. These systems can keep you from making copies (or sometimes even one copy) of an HDTV program.
The most common copy-protection system is HDCP (high-bandwidth digital content protection). This system is in new HDMI-equipped devices and many DVI-equipped devices. HDCP encrypts (scrambles) the content sent between devices like tuners and TVs. This encryption is a problem if you have a DVI connection where one piece (like a set-top box) uses HDCP and another (like your HDTV) doesn't. That's because the content owners have rigged the system to down-res (down-resolve) HDTV programs to standard-definition unless HDCP is present at all points in the system. You could end up unable to get an HDTV signal on your HDTV!
If some but not allof your HDTV devices have HDCP, component video might be the only way to get a true HDTV signal between your devices. Component video isn't limited by HDCP, because component video isn't a digital signal.