I don't get it. I have ALL my Kindle books on both my Kindle and in my Kindle4PC, and a lot of them on my Ally phone, so at least for me it just isn't true that DRM'd books can only be opened on one of your devices. I just had to register all of my devices to the same account/user. Until I started reading this I didn't realize it was even a problem for anybody. What could be the difference?
How do you sync between devices and read part of a book on your phone and part on your Kindle if you can't do this? Since that is one of Amazon's selling points I'm sure it's supposed to work that way for everybody. This is what my "Manage my Kindle" page looks like, with the dropdown for "Darkly Dreaming Dexter" showing. I can at any time send it to any of those three choices. (Sorry, I couldn't figure out how to make a screenshot into a jpg.)