I never knew this up to now, but TEXCOORD inputs to a pixel shader have an optional centroid modifier
since shader model 2.0.
Since a pixel shader is only executed once when MSAA is enabled, it allows you to choose your interpolation style. By default, the input values are interpolated for the center of the pixel. With the centroid modifier the input values are interpolated for the centroid of the active MSAA samples. This image explains it all:
So, to interpolate as shown to the left, you'd use:
float4 myInput : TEXCOORD0;
To interpolate as shown to the right, you'd use:
float4 myInput : TEXCOORD0_centroid;
Also good to know is that while centroid sampling is an option for TEXCOORD inputs, it is always enabled for COLOR inputs. It's a small difference, but it might help get the most out of your shader with MSAA enabled. A little Tip and Trick in the HLSL and Shaders board.