PRIME is the project that originally came about in the X.Org world for supporting technologies like NVIDIA Optimus with buffer sharing and GPU offloading. DMA_BUF is the Linux kernel infrastructure that allows zero-copy sharing of buffers between kernel drivers, such as from one DRM driver to another (so multiple GPUs from different vendors can share a buffer in a standardizes way) or from a DRM driver to a driver in a different sub-system (e.g. V4L2; namely in the case of ARM SoCs).
The work that Kristian Høgsberg published on Wednesday evening is support for creating a DRI Image from a PRIME/DMA_BUF file descriptor. The patch series then goes on to add support for wl_drm buffer sharing via this file descriptor passing.
This work isn't for Wayland/Weston itself but to the DRI2 EGL driver, the Mesa DRI driver to support creating a DRI Image from a passed file descriptor, and the EGL Wayland DRM code living within Mesa.
The set of three patches can be found on the wayland-devel list.
In case you missed it, earlier this week Kristian also proposed pointer locks for Wayland that are modelled after the HTML5 pointer locking interface, which is primarily useful for gamers.