@Setanta,
My SIL makes the dip Morgan is talking about and she uses Velveeta -- melts it in a crockpot and adds a pound of crumbled ground beef and a can of Rotel tomatoes. It's soooooo good lol. We tease her a lot about using the processed cheese, though.
I've made it by first stirring up a roux and then adding the cheeses slowly 'til they melt -- then the tomatoes and beef. My SIL's method is, of course, way quicker and just as tasty (if not moreso).