From the Pottermore Insider (Official Blog):
We’re pleased to announce that pottermore.com will be open to everyone in early April 2012.
We know that the extended wait for those wishing to be part of Pottermore has been frustrating, and we’d like to thank you all for your patience so far.
Many of you are interested to know why we extended the original opening date beyond October of last year.
We always knew Pottermore would be incredibly popular, which is why we made the decision to only open to one million Beta users to begin with. We wanted to make sure that we had a really good understanding of how people want to use the site and which bits we’d need to modify before giving more people access.
We gathered some incredibly useful feedback from our Beta users, and it became clear that our original platform wouldn’t be suitable when millions more users came on to the site. So we made a big decision: to move Pottermore to an entirely different platform set up. This ‘invisible’ change has involved a lot of work behind the scenes but it will enable our users to get the best from Pottermore as it grows and develops.