Another story indicating that Amazon will provide an ePub version in
this article. However, it doesn't site any sources and appears to just be some more speculation.
I think there is a good chance that Amazon will provide ePub support. Here is my reasoning--
Rowling is selling the DRM-free eBook on her website to cut out the middleman (e.g. distributors like Amazon, Apple, etc.). It has to be DRM-free because she doesn't have access to information needed to encrypt the file for each device that will buy the eBook. However, she has found a clever solution to providing a book that isn't encrypted-- add a watermark to the book that makes it clear who is the book's owner. This is not too difficult to accomplish with ePub, but I don't believe it can be done in the mobi format.
Amazon won't make any money when people buy Rowlings' books through her website, so they really don't have much incentive to start supporting ePub. However, the lack of ePub support will be highlighted in a major way if the Kindle is the only major eReader that people can't use to read the Harry Potter books. Amazon does NOT want to see this pointed out in an ad for the iPad this Christmas. So... I think Amazon may start supporting DRM-free ePub. This allows them to make the Harry Potter books available, while at the same time still locking out the DRM'd ePub books of their competitors. Since Amazon has said they are working with Rowlings' people to make the books available, I think there is a good chance this is what they're doing. Amazon is also expected to be releasing a couple new devices soon, and once again the lack of ePub support will be a glaring deficiency-- especially if they can't read the Harry Potter books.
Yet, I've been spectacularily wrong in the past.