For this you will need a friend. (Or you can solo if you're VERY well geared.)
you will need a plate wearer to do this.
Go to icecrown, Cathedral of Darkness. (just west of ICC)
There will be about 48 elite mobs (37k hp each if i remember right) and they pull 1 side at a time (if you aggro one, the whole side will aggro). Therefore you pull 24 mobs at the same time.
My friend is a tailor, so he got alot more cloth than me, but it works even if you're not a tailor.
The mobs instantly respawn as you kill the other side. IE. you kill the left side, right side will respawn (and vice versa)
in about 20 minutes we had more than 350 cloth combined. + gold from looting + greens / grays
enjoy and be creative what you can do with the cloth