There are several official releases from the launch of Pottermore: JK Rowling's video, the press releases, etc. None actually mention the specific formats that Pottermore will sell. (While Rowling did mention ebooks in her video, she did not say Epub, Kindle, or any other specific format.)
But there are reports on what Pottermore will support. Wired UK have a pot saying that the Kindle ebooks will be sold, and I followed up with that journalist. She said that she heard about the Kindle support at the press conference. She was the second person to tell me this, so I will take it as fact. And then there was a story posted on CNN Money, and that article said basically the same thing. That article's source was with Bloomsbury, publisher of the Harry Potter series.
I'm sure you're thinking those are ironclad sources, but here's my problem. I followed up with Pottermore, and I could not get their marketing people to confirm any details. I asked about DRM, format support, and other general ebook stuff, and I was told:
Ebook details won’t be announced until closer to the site and shop opening in October.
I don't mean to imply that anyone posted bogus info, but I also don't know that they were given the correct details. If this really were a settled issue, then wouldn't the marketing people have the answer?
And doesn't today's announcement make you wonder? I mean, if Pottermore were going to support Kindle format, wouldn't they have put out their own press release stating how ebooks will be handled?
If anyone cares to go bug the Pottermore people and push them into making a definite statement, be my guest. Right now, ebook support is still a question, not a fact. We don't know for sure that the Kindle will be supported. I think the answer is yes, but I don't actually know that it will be.