OK. So we are (happily) "out of a closet". HBG's David Cramer gave a presentation at BEA 2012 in which IGP:Digital Publisher was mentioned. IGP:Digital Publisher has been used for the comprehensive production of a wide range of best sellers by HBG over the last few years; in multiple print formats and e-book deliverables.
Dave is the HBG digital content guru, and a significant contributor in the IDPF ePub3 spec. He co-authored the fixed layout spec.
HBG is not the only one, but we have to respect NDAs (Non-Disclosure Agreements) when they apply. Others users include Faber in the UK and Oxford University Press for Online interactive content and audio readers. There are also a number of other quality publishers around the globe who understand the difference map between digital content and format outputs.