NVIDIA today confirmed with Legit Reviews that they will be supporting the DirectX 12 API on all the DX11-class GPUs it has shipped. This means that all cards using Fermi, Kepler and Maxwell GPU cores will be supported along with all future GPUs.
NVIDIA was quick to point out that only 40% of AMD Radeon video cards will support either DirectX 12 or their own Mantle API. This is because AMD has announced that for the time being, only Graphics Core Next (GCN) enabled products will support either. This means that many of AMD's DirectX 11 cards will not be supporting either. Legit Reviews spoke with AMD at GDC last week and they said they aren't sure exactly what will or won't support DirectX 12 as they haven't fully made up their minds yet. It will be interesting to see how this plays out!