ah, i didn't think about the proprietary bit... you might be right not to mention that, although i do think InDesign is becoming a bit of an industry standard as well so it might be another point to convince publishers that it won't be too much work to add it. perhaps we could say "the EPUB format can be exported from some major professional layout and design applications, so it can easily be integrated into the publishing workflow."
and definitely key to mention open source !!! can't believe i forgot that. very important.