This is so much fun! Saw a tutorial on CraftGossip and had to give it a try. So simple and very stunning.
I started, then got called away, and found that the ink carries differently over already dried spots. I could play with this method for a long time trying to master it, but the fun is - you don't have to! I do, however, recommend doing this in a well ventilated area.
It washed up really great too. I did heat-set it, with an iron and in the dryer for 30 minutes on high heat. Then washed it in cold. Next I'm curious to see if I can use laundry soap or crayons to create a batik-type relief for a pattern. Could be very cool. Stay tuned!