Have you checked out the Kindle page on Amazon.com lately? If you have then you might have seen this:
So what do you think it is? Laura followed up with Amazon and they teased her with: “We’ll have to ask you to stay tuned for an upcoming announcement.”
It's on the Kindle homepage so clearly there's going to be some type of HP/Kindle tie-in or bundle. You can only buy HP ebooks via Pottermore, not directly from Amazon, so a content bundle is unlikely. But this teaser might be an indication that the Pottermore exclusive ebooks might stop being quite so exclusive. The Harry Potter Encyclopedia is also an unlikely connection, given that it is a couple steps back in the pipeline behind JK Rowling's other books.
So my guess at the moment is a Harry Potter branded Kindle. It would likely come with a cool case and possibly a skin, and it might even include a coupon that you can redeem on Pottermore.
In any case, I'm looking forward to finding out.