Dolby's latest surround sound entry, Pro Logic IIz, introduces two additional channels to the ever growing mix - height channels. This expands the audio stream to 9.1, which means three speakers across the front, two on the sides, 2 in the rear, a subwoofer reproducing the low frequency effects channel (the ".1" of the 9.1), plus the new stereo height channels. The height channels are feed into speakers located in the air above the listener, giving a sense of spaciousness that can't be had any other way.
Height channels are nothing new actually, as they've been around since the beginning of modern surround sound itself (1993 to be exact). Many audiophile mixers, having no use for the .1 low frequency effects channel, used the full bandwidth channel as a height channel for some recordings. Good idea but it never caught on (but surround sound didn't either).
The problem with height channels is that many times it's difficult to mount the speakers in the correct locations up in the air (so what else is new?). But a bigger problem is that the standard position of the side speakers in a 7.1 configuration still isn't settled. Not to mention that there are few recordings available in 7.1, and very few consumer setups at home. 7.1 is having a tough time gaining acceptance, let alone 9.1.
Having played with a height channel for about 10 years thanks to recordings made with the early Holophone surround microphone, I can tell you that it does add that extra realism that you can't get any other way (if realism is what you're going for). But the hassle factor is large for the production team, and still a huge barrier in the home. Most people are just fine with stereo, unfortunately. Hats off to Dolby for pushing the technological envelope though.