I expect all devices in the future to be able to read the same standard formats... that's easy. But the issue is much more than just which formats the device reads... it's in how they are presented.
A recent
TeleRead thread discussed preferences between spaces and indents between paragraphs. Here, we've discussed issues like display margins and contrast issues regularly. The winning devices will be those that allow the user to set their display options they way they like them, to optimize their reading experience. Software flexibility... that's what will win consumers over.