MythTV developers are dropping the X-Video Motion Compensation support since the code is hard to maintain and it's simply not worth the effort. X-Video Motion Compensation is not too advantageous with modern processors and graphics hardware. VA-API and VDPAU are the premiere video acceleration methods under Linux at this point with XvMC being rather cruft and never received its improvements. XvMC support these days on Linux is also rather limited to just the Intel graphics driver, the VIA driver, and select other ASICs taking advantage of the XvMC Gallium3D state tracker that uses GPU shaders for decoding. There isn't that much more support beyond that.
MythTV already has support for VDPAU in conjunction with the NVIDIA GeForce 8 series GPUs and later running NVIDIA's proprietary driver. This is the optimal solution for MythTV with the Video Decode and Presentation API for Unix allowing for HD video playback with a $20 CPU and $30 GPU under Linux.
The MythTV developers are also working on support for Broadcom Crystal HD, Intel, and ATI support to complement the NVIDIA video acceleration. While the MythTV developers didn't comment on what video API they will be targeting for the Intel/ATI support, it's safe to assume it will be VA-API. VA-API is supported by Intel's latest Core i3/i5 hardware and indirectly on the ATI/AMD side by the closed VA-API to XvBA library since the X-Video Bitstream Acceleration API itself is undocumented and is not implemented anywhere else besides within this library that was written by Splitted Desktop Systems in cooperation with AMD.
The MythTV developers will also be dropping support for libmpeg2 on the basis of bugs and problems its causing for the MythTV project. Along with XvMC, the MythTV developers are also looking at dropping X-Video support too, but that may not come until MythTV 0.26 or later.
X-Video is supported by more open and closed-source Linux drivers drivers, but it is even of less benefit than X-Video Motion Compensation. The X-Video extension has been around for nearly two decades and within the MythTV project for those not using VDPAU/VA-API/CrystalHD, it will be replaced by a better OpenGL video playback renderer. MythTV developers are also looking at supporting OpenGL ES too for a better MythTV front-end experience on embedded/mobile platforms.
The announcement was made via their project blog.