Interference from other devices can be a problem with electronics, including HDTVs. When you run a varying electrical current through a wire or circuit, it can make radio waves. (Just hold a transistor AM radio near a computer if you want proof.) And these waves can create electrical currents when they hit other wires. This can be good — as in when you want to receive broadcast radio signals — but it can cause problems when it interferes with cables that are carrying other information.
Gefen has announced that they have created HDMI and DVI cables that use fiber optic cable to make the connection. This means that the cables cannot transmit any radio-frequency emissions, nor can their signal be interefered with by outside sources.
Frankly, this sounds like overkill to me. If you’re working with government secrets or some other sensitive application, and you need Tempest-style protections against electronic evesdropping, then it makes sense. I doubt that interference can make a significant effect on HDMI or DVI video connections, and that a good-quality shielded cable along with minimal attention to avoiding big electrical motors and such will probably be sufficient for most installations. I do expect, however, as the cost of fiber optics inevitably falls, we’ll eventually see all copper cables get replaced by fiber optics.