here's my experience: 03' sierra 2500HD, took it into dealer(june 08) for leaking trans.cooler line,told them i'd wait for it. Service advisor comes to waiting room an hr. later and says that the line will be replaced and while inspecting the vehicle they determine all the ball joints are out of spec. Said they made a call and they are covered under my extended warr. and would i like them replaced. Naturally, i said yes. Well, when the truck is finished i'm looking at the paperwork and see where all the control arms are also replaced.I asked what was wrong with them and was told the the ball joint/arm are a pressfit unit and will only be covered by replacing the entire assembly. From what was explained to me was that if just the ball joint is pressed out it stretches the socket hole and pressing a new joint in doesn't always hold up and they can come apart. Up here in my neck of woods the control arm assembly is around $300 apiece at the parts stores.
Forgot to mention: replacing control arms also requires a front end alignment,glad none of that expense came out of my pocket that day!