E-Reader-info just posted an interview with the head of Bookeen. Here is an excerpt:
Q: Your readers use the Mobipocket DRM files, or alternatively ePub/PDFs. Why can't these two live together?
Mobipocket belongs to Amazon. Amazon forbids us legally to have two DRMs on the same device. Technically we can do it, legally we can not.
Q: Most readers seem to use either Adobe's files or ePub. Why did you choose Mobipocket?
This question is an historical mistake.
Adobe EPUB has appeared 18 month ago. We have heavily promoted the EPUB format even before most of our competitors were thinking about eBooks. We were the second reader to be Adobe EPUB/PDF certified just after Sony. And before that we had even our own EPUB software reader.