Back In the day i took me a few days to make texture splatting with opengl and the fixed function pipeline.All those sampler states,texture units blending arguments etc was just pain in the ***.
Well it took me only a few hours to make the splatting work.It is very primitive at the moment but It works.The vertex shader passes the vertex normal to the pixel shader and the pixel shader interpolates between two texture depending on the vertex normal.
I'll add 4 texture weights to each vertex so I can blend between four textures in the shader.
I tested the application first on an old radeon x550 which i have at work.Well It was running at ~40-50 fps.Which was a big drop from 100.So I think x550 was sucky at shaders.I tried it at my geforce 7600gt which is not a great card today but it can kick at 300 to 500 fps :).So I guess performance is okay.
Here is a screen shot :