After VP2 video decoding support was added for H.264/MPEG2 video acceleration support in the Nouveau reverse-engineered NVIDIA driver, it was realized it wasn't too much more work implementing the VP3 and V4 video engines. These blocks that are part of NVIDIA's "PureVideo" technology have now been implemented in Nouveau Gallium3D for accelerating MPEG2 and VC-1 codecs.
There's also H.264 and MPEG4 support for the VP3/VP4 engines, but that support is disabled on pre-Fermi hardware in Nouveau due to bugs. Today's VP3/VP4 enablement is for the NVIDIA "NV50" class hardware, or more easily known as the GeForce 8 through GeForce 300 series products.
The commit that went into Mesa today for enabling the VP3/VP4 NV50 support can be found on cgit.freedesktop.org.