I just found out today, from a newspaper article in the Santa Cruz Sentinel, that there are now companies doing custom-printed fabrics. You design your repeat block using whatever tools you like to get a raster image (like GIMP or Photoshop).
The finished block is then … uploaded to an on-demand digital printing service, which can print on all sorts of fabric in quantities of your choosing. Heather uses Spoonflower, which prints swatches, fat quarters, and yards of fabric, plus wallpaper, peel-and-stick wall decals and wrapping paper. … Another site, Fabric on Demand, specializes in custom fabric printing, and offers some fabrics, such as lycra/spandex, not available through Spoonflower.
Both websites are user-friendly, offering several design layout options, step-by-step instructions, and examples. The process is not inexpensive, though. On Spoonflower, for example, cotton fabric starts at $17.50 a yard, and their most expensive option, silk crepe de chine, is $38 a yard.
The prices are pretty high, but it sounds like a cool way to get custom-printed fabrics. (My interests in fiber arts were in weaving, not sewing, so I don’t really have any use for custom-printed fabrics, but the idea is cool.)