The Pottermore books use some "watermarking". Have you taken a look inside the epub to see what they're actually doing? Run it through calibre conversion epub-to-epub and see what happens?
I much prefer watermarking over DRM. It's the right evolution of the market, as seen previously with digital audio, and I don't really want to discourage this approach since we all know DRM sucks. I'm just really curious how the watermark ended up being done.
Edit: Never mind. Found the discussion in the News forum. I'm apparently very late to this news
