The speculators were right, Basic D&D 5e will be a 96 page download from the Wizards of the Coast site, available the same day as the Starter set, July 15th 2014.
Interestingly enough the cover has a party of adventurers in a mysterious place, against mysterious foes.
Here is the official description of the product.
This is a great move on WotC's part. The only mystery is why they didn't just explain this when they announced the Starter set, avoiding a lot of fuss.
Sure, I'd still prefer the Starter set be complete, however there is an advantageous side to this method. They can release larger adventures and just point to the site for the rules needed which means more room in the printing budget for adventure materials.