EPUB is supported on all of Sony's Reader devices (including the older PRS-500 if you send it in for a firmware upgrade), the Barnes&Noble Nook, and almost all of the other available eBook readers. There are good epub readers for MacOS and iPhone (Stanza), Android (Aldiko), Windows (Adobe Digital Editions), and Linux (FBReader, Calibre). The only notable exception is Amazon's Kindle which supported the .mobi format already available. It's also possible that industry pressure could eventually force Amazon to support the format as well. Sony is moving their eBook Store entirely to epub next month.
EPUB is backed by the International Digital Publishing Forum (IDPF, the standards organization for the digital publishing industry). At it's core it's based on XHTML and CSS. It supports complex layouts, images, re-flowing (for different sized devices) and generally most of what HTML supports. It also supports font embedding, DRM (Adobe, but DRM would greatly limit which devices could be supported), and a real table of contents structure with parts, chapters, sections etc..
Supporting EPUB, even for something as complex as the scriptures, should be fairly easy since it's based on XHTML. An XSLT could be written to do most of the heavy-lifting from the XML or HTML used internally by the church.
Also according to this post: http://tech.lds.org/forum/showthread.ph ... =PRS-500#9 tomw owns a PRS-500.
