In a large saucepan, melt butter; stir in flour until smooth. Gradually add milk; stir in the salt, pepper, mustard, Worcestershire sauce and nutmeg. Bring to a boil; cook and stir for 1-2 minutes or until thickened. Stir in cheeses until melted.
Drain pasta; stir in cheese sauce. Transfer to an ungreased 2-quart baking dish. Bake, uncovered, at 350 degrees for 20-25 minutes or until bubbly.