Is there any shader with the functionality of a displacement map yet? Or has anyone posted a patch or a node like that already? Or is there a not too mathematical or complicated way to build such a thing with the given nodes?
the User Shaders page is supposed to collect this stuff. There is a deformation shader which should be close to the shader you are looking for.
basically it should be a good introduction to shaders to built your own - read the pixelshader_for_newbies tutorial first.
just modify one of the template shaders:
technically you would need to add another texture input pin. in the pixel shader you would need to do another tex2d texture lookup and use the resulting red and green values to add an x and y offset to the coordinates of the texture lookup which is already there.