It's still unknown whether this is exclusively for the NVIDIA Founders Edition models. There are rumors that the custom-design cards will not require this 12-pin connector.
It's expected that NVIDIA will bundle an universal adapter with its Founders Edition cards. The model shown below from Seasonic is reportedly only compatible with Seasonic PSUs. Interestingly, the PSU maker recommends using this cable in combination with a 850W (or higher rated) power supply.
Some websites like the German HardwareLuxx have also posted pictures of this cable, indicating that it's already circulating among reviewers.