Sept. 25 is a Tuesday, which is one of the preferred days for patching and other fun. The release date is a bit later than many expected, but Blizzard operates under the philosophy of "it's done when it's done," so a later date only means we're getting a product that's been through a lot of testing.
You can upgrade to the standard edition and digital deluxe edition through your Battle.net account page now.
Update 9:30 p.m. EDT: Added the official press release announcing the release date. You can view it after the break.