It's fairly well-known that for some time music executives and Apple CEO Steve Jobs have clashed over the pricing structure for music on the popular iTunes service. Apple refuses, for example, to change the price for DRM'd music from $0.99 per song, and won't pay royalties on iPods sold to music companies. In general, Apple is bargaining from a position of extreme power as it's currently the #3 music retailer in the US (including brick-and-mortar retail locations) so music companies either accept the terms, or face losing a significant number of sales.
Well, Universal Music has decided to call Apple's bluff. They have refused to renew their contract with iTunes. Now, don't worry that suddenly you won't be able to buy any Universal Music songs via the service. What this means is that Universal Music tracks will continue to be sold through iTunes, but that Universal, lacking a contract, can pull its content at a moment's notice.
iTunes is the #3 music retailer, and Universal Music is the largest music company in the world. In this pretty big game of chicken, who will blink first?