The e-learning industry is an enormous, decentralized and definitely still very heterogenous one. There are no definitive go-to sources, yet, even though some players in the industry like Apple are shifting large amounts of money into R&D&M (research and development and marketing) to change that.
You are asking about ePub becoming the de facto standard for e-learning. In fact, there are so many standards and formats out there still, that I think the consolidation process of standards has just begun.
First of all, ePub is of course only highly relevant for text publishing (you might want to think about rephrasing your question), and e-learning consists of all media types. For instance, large amounts of media out there are either mixed-media or video.
Secondly, format standards or standard consolidation depends highly on the publishers and distributors. For now, it seems that most of them are still trying to diversify their format portfolio rather than to consolidate it. Apple iTunes U is a good example, providing support for multiple video and audio formats, as well as PDF and ePub text publishing (source: http://deimos.apple.com/rsrc/doc…).
Finally, all e-businesses are innovation driven businesses, and ePub still has a way to go before it is able to fully compete with more established (not open) formats in every necessary way.
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