Update: HP ebooks are now live in the Kindle Store and the Nook Store. The links in those stores redirect to Pottermore. I've just bought one and I was able to transfer it to my Kindle account on Amazon.com, and I strongly urge you to do the same. Pottermore limits you to only 8 downloads. Other options include transferring the ebook to the Nook Store, Sony, and Google Play.
The ebooks will eventually be available for purchase via the Kindle, Google Play, and Nook ebookstores, but not iBooks. From the WSJ description, it sounds like you will be able to buy the ebooks in the respective stores and download your copy from there. That's going to be nice for Nook owners, but it doesn't have much appeal for Kindle owners, now that we have the Kindle Cloud to store our ebooks. There's no word yet on the library editions, but the last I heard they will be available before the end of April.
Rowling's books have sold an estimated 450 million physical copies and been translated into more than 70 languages. I'm sure she's going to have similar success with the ebooks. Pottermore has just left its beta period and opened up to the public, and it already has millions of fans.
I have to ask: are the price for real? They're asking $8 for books that were originally published starting 15 years ago. I don't even pay $8 for a book published last year.
While I'm sure they're going to sell quite a few copies, I'm also pretty damn sure that the high price will turn away a lot of parents. These are folks who have probably already bought at least one complete set of the paper editions for their kids, as well as the movies, games, and other junk. My mother, at least, would baulk at buying an expensive digital set as well, and she likes books.
I also have to wonder if this is really the best way to introduce the first legal copies of what has to be the most pirated ebook in recent history. Up until today, there were no legal HP ebooks. That guaranteed that anyone who wanted one had to go get it free from an illicit source.
So that means that most everyone who wanted ebooks bad enough already has them. Pottermore's task was to convince those pirates to go legit. Oh, I think they should replace the pirated copies with legit ones; that's the ethical thing to do. But given the high ebook prices, how many pirates will decide it's not worth it?
The real story today is that JK Rowling is still leaving money on the table. She ignored ebooks for many years and is now selling them at a steep premium. Clearly she doesn't understand the ebook market anymore now than she has in the past decade.