I had argued last week that Amazon wouldn't adopt Epub because they hadn't adopted PDF. Well, it turns out that they have, and that negates my argument.
The KPR file is a wrapper for a PDF file. Now that Amazon will also let you open a PDF in K4PC, I do not think the wrapper is an attempt to disguise the PDF. No, it's simply how Amazon added their own DRM on top of the PDF file. If and when Amazon adopt Epub, they still won't admit to selling you one. But there's a good chance that they will let you read your own (DRM-free) Epubs. That is a close second to my desired outcome.
But before you get too excited about Epub, note that Amazon adopted PDF because they wanted to go after the textbook market. I don't think they will adopt Epub unless they have a similarly really good reason to do so.
On the other hand, the library ebooks support (that the Kindle is going to get this fall) could be the reason for Amazon to adopt Epub. Who knows, we might wake up one day next month and find that Amazon have turned the world upside down again.