The USDA does still grade beef, but it is almost meaningless. As far as I am concerned USDA stands for United States Department of Agri-biz. USDA grades do not insure quality of the beef. The grades mainly describe the tenderness of the beef by a visual check of fat content and distribution. This is something you can do yourself at the supermarket.
The most common grade you will find at a supermarket is USDA Choice, which is not bad. The meat should have most of the fat integrated into the meat in very visible veins and have few visible chunks of fat. It is still a high fat product though. USDA Prime is even higher fat with better marbling, but you will almost never encounter it in supermarkets.
Rather than labeling each individual cut in the supermarket there will be a sign somewhere stating "we sell only USDA Choice Beef."
In short the USDA grade only tells you about fat content and how it is distributed in the meat.
Confusion does abound in supermarket beef though. "Certified Angus" does sound nice, but only means that it is certified by the beef industry that it comes from a specific cattle breed that some people find tasty and others don't. It says nothing about overall quality.
You will also see "natural beef" sold upscale. All they guarantee is that unnecessary hormones and antibiotics are not in the beef.
As far as finding USDA Prime in a supermarket, good luck...it is usually sold to upscale restaurants and specialty stores.