That means PlayStation 3 begets a PlayStation 4 download and Xbox 360 in turn unlocks an Xbox One version. What's more, each console will retain its respective license and you'll still be able to play online with your buddies on new and old boxes (with frickin' lasers!) after you do the deed.
The offer expires at the end of next March, and like with Destiny before it, premium content like season passes will carry over too. Sounds like a pretty great deal unless, of course, you were planning to pick up one of those special edition Xbox Ones.