Tony Iommi of Black Sabbath tuned his guitar an octave low because of him losing the tips of two fingers in an industrial accident back in Birmingham the mid 60s. Without that accident we would have never heard two of the greatest guitar licks of all time, "Iron Man" and "Paranoid." 
Uh guys, I hate to ruin a party here, but tuning a six string guitar an "octave lower" would make for a terribly sounding and playing guitar. An octave is the interval of eight diatonic degrees between two tones of the same name (like a low E and a high E), the higher of which has twice as many vibrations per second as the lower. Six string guitars are not made for reproducing such low tones correctly although you can play around a lot with a seven or eight string guitar and achieve some success tuning down an octave.
A four string bass guitar for all practical purposes is tuned one octave lower than a normal guitar, top string being an E, but it is set up with a longer neck and bigger strings to accomodate that tuning. Were you to do that with a six string guitar your lower strings would practically be laying on the frets.
Don't you mean tuning down a "step" or two? That's what Tommy Iommi did. He tuned down three half steps from an E to a C# when he was with Black Sabbath (starting with 1971's Master of Reality, well after he recorded Paranoid and Iron Man...sorry ). Tuning from an E to D is called "drop D tuning." The lowest I've ever heard of guitar tuned down and recorded was tuning down from an E to a A by the band Korn and several others.
Seilerbird could tune lower than an A but he would have to change strings to a larger set, adjust his Floyd Rose, and hope for the best. And we know how much Tom loves changing strings...
