Spoonflower, seems like such an awesome new site. You upload a design and they print custom fabric for you. I really want to try it out. It costs $18/yard, but you can get a swatch for $5, so I think you can try it out before committing fully.

The fabrics above are by Spoonflower user Katherine and Strawberry Luna. You can check out some more of the fabrics people have made at the Spoonflower Flickr gallery.