World Coordinates of face on a mesh

How do you find the world coordinates of (the center of) a face on a mesh? How do you reference a particular face on a mesh?

It depends on what you're trying to do, but generally speaking, you would access the mesh data via the Mesh class (or possibly just access the original data directly if the mesh was created procedurally), and then transform the desired point using (e.g.) Transform.TransformPoint().

Beyond that though, I think some more detail is probably needed.