Seeing as how this good old thread has just been re-revivied, let me correct something I got wrong back in October 04: Jimi Hendrix did not play "upside down and backwards" as Libba Cotten did.

I recently rented "Woodstock" on DVD, checked out the closeups of Jimi's right hand, and saw that he played "conventionally" for a lefty, with the strings installed across the guitar fingerboard in the opposite order from the usual "right-handed" arrangement. He was quite obviusly in standard tuning, and his chord shapes were the exact mirror-image of how a right-handed player would make them.

Elizabeth Cotten's approach was something else entirely. Her guitar was strung normally, but she flipped it over and fretted with her right hand and picked with her left (with the left thumb positioned over the treble strings and left index finger over the bass strings); in other words, she did play "upside down" as well as "backwards."