I could see this becoming very popular as a heart-rate etc. monitor at high-end gyms. Slap on a tattoo that will interface with whatever machines you use, then wash it off in the shower.
Nice, but how scalable is the fabrication process?
To answer my own question, there was an article in Science yesterday which goes into much more detail on this project. The backing appears quite scalable if there aren't any hidden gotchas that they didn't discuss. The electronics are probably the tricky part, but they still look doable.