For me, I always think of desert nomads of some sort when I hear 'dervish.' And whenever I think of desert nomads, I think of the Fremen of Dune.
'Walk without rhythm, and you won't attract the worm.'
The player has to make a series of dance checks on sand(low dc, 10?) in front of a purple worm as they try to get between two points.
If they get the same number twice in a row, they walked with rhythm, and the worm attacks. If they survive without being eaten (whether they cut their way out or not), they learn the secrets of the dervish.
One of the later Dune books has the citizens of Dune dancing for hours in front of a sandworm without it attacking as a test of their skills. I think it was the one before Chapterhouse Dune.
Oh yeah. Watch this. http://www.youtube.com/watch?v=sMZwZiU0kKs