Another guy (using the "ZOSO" symbol as his avatar) just came on there and said that yesterday he saw an interview where Jimmy Page said he played it in Open G on a 12-string.
Standard is the same as Open G on strings 2-4, so it wouldn't be too hard to "fake it," but there are some slide double-stops you'd miss that way, that fatten up the sound. Some people "fake" Open G by just dropping the first string from E to D. Sort of a reverse Drop D.
The bizarre tuning suggested in the widespread tab is EACFAC. That makes no sense to me at all, and it's not even something I've been able to find on a bunch of extensive lists of alternate tunings. Google doesn't turn it up for anything except that tab. Seriously, sometimes I wonder if the music publishers don't hire people to write and propagate bogus tabs, as the RIAA is well known for doing with corrupted MP3s on the file sharing networks.
Have you ever listened to Memphis Minnie's original of "When The Levee Breaks?" It's a bouncy, happy dance tune. The music seems kind of out of touch with the lyrics.
But, in the sage words of R.L. Burnside, "The blues ain't nothing but dance music anyway."