NOT be opened to the public until late October.
The Pottermore Insider says:
"From the end of October, registration will be
opened to everyone and we’ll be giving access
to registered users in phases.
Access may be granted quickly, but please note
it could also take some weeks or months,
depending on demand.
Also: "...we’ve decided to extend the Beta period beyond September and take
a different approach to the way new users are brought onto Pottermore."
So if you've been one of those granted early access, then you have
one more month to enjoy the site exclusively.
Please note that the notice said this clever release date hint:
"From the end of October, registration will be open to everyone."
This encourages the rumor of Pottermore opening on October 31st,
the day that Harry Potter got his lightning shaped scar.
Is this a coincidence, or is it hinting a release date?