The folks at Pottermorethat the site will officially open to the public some time in April 2012. They've learned a lot from the beta test that they've been running for the past 7 months or so, and they figure that by April they should be ready to expand beyond the first million users.
But I don't care about that; I want to know when the ebookstore will open. Their blog post doesn't mention the ebookstore, so I followed up and I was told that:
The shop will open at a similar time in early April, no specific date or partner details as yet.
You heard it here first, folks. The first legal Harry Potter ebooks will hit the market in just over a month. Surprisingly, the HP library ebooks won't be available until the end of April (according to Overdrive), even though they were announced last month.
I haven't seen it myself, but Pottermore is reported to be the hot new community for Pottermaniacs. You know, people like this: