Wizards of the Coast has released the trailer for Theros.
Theros will be released on September 27, 2013 (October 7 for MTGO). It features 249 cards, and is the first of 3 sets to be released in the Theros block. Prerelease events will be held September 21-22, 2013 (October 4 for MTGO).
If you want spoilers, MTG Salvation is the place to find them.