Ping really seems like Apple is trying to revive it's Genius tool. Genius was *supposed* to be this great recommendation tool, quickly making playlists and suggesting new music based on what other people listened to. Only it sucked and suggested the crappiest crap, usually based on music that was so base and common there was no point in even researching it, let alone actually listening to it.
Now Ping is based on social networking, or letting you tell iTunes who your friends are so they can try to make suggestions on that.
Ultimately this sounds like Apple is still trying to catch up to freakin' Amazon's "People also bought" and "Recommendations" database. There are so many tools for finding similar music, from Pandora to Last.fm and more, that a feature that will require a lot of effort for no payoff will fall flat. Social networking works when there's little effort involved, which is why Facebook works so well (you can get a lot from Facebook with only an account and a single friend, since others will friend you for you and you just have to click a few times). Something that will require you to track down friends in a closed system? Pfft. Maybe if iTunes had a web-based platform, but I never use the store anymore because I can't even shop for music in different tabs. It's the slowest, most annoying way to buy music online, and Ping isn't going to do much to change that.