Then we have Adobe's Digital Editions, which uses ePub for layout. Adobe also provides a PC client, and manufacturers of eBook readers can pay around $75,000 for a licence as long as they also agree not to include Mobipocket or Kindle software on the device: Adobe wants the field to itself.
Funny, my understanding is that it's always been the exact opposite
-- Mobipocket/Amazon wouldn't let you license their DRM (not that they're even licensing it out anymore) if you wanted to use it in conjunction with any other company's DRM. From what I know, Adobe doesn't have that stipulation. Oh wait, silly me. It's the Register. I should've known that they wouldn't get things right...