This looks like something I saw on Nichole Heady's blog quite a while back - you just stamp your image in dye ink and then go over it with a plain waterbrush (water only) to get the lines of the image to bleed out a little bit. It's a really neat effect with not a lot of effort.
After some SERIOUS searching, I found the blog post where she used it. It's not really a tutorial, but she does have a couple of pictures that should give you the general idea:
Watercolor Bleed