video hardware acceleration is provided through:
* a v4l2 kernel driver
* a Gstreamer plugin (v4l2dec and v4l2enc)
In order to use h/w acceleration, the application would need to use one of these components. So it should be "easy" to get any Gstreamer based application to benefit from acceleration. mplayer is not based on Gstreamer, so it won't be using it.
this was about codec operations.
for video rendering most application should be able to leverage the GPU through OpenGL/OpenGLes.