Patches are pending to implement support for NVIDIA's VDPAU interoperability OpenGL extension within Mesa/Gallium3D.
The pending patches implement the NV_vdpau_interop so that VDPAU video and output surfaces can be used for texturing and rendering. Decoded VDPAU content can be passed to OpenGL to either display or further modify these video surfaces.
The NV_vdpau_interop OpenGL extension really opens a lot of new possibilities for multimedia programs using VDPAU for accelerated video decoding. NVIDIA's binary driver has implemented this extension for some time as part of their Video Decode and Presentation API for Unix, while now it's coming for Gallium3D. The extension is outlined in the specification at OpenGL.org
One of the notable applications that is able to take advantage of this VDPAU OpenGL interoperability is the XBMC media player. The extension is for Gallium3D drivers that support using the VDPAU state tracker. The patches until they've been merged to mainline Mesa can be found on the Mesa developers' list