The DirecTV listing has since been changed to remove any reference to Ozzy, however it appears they jumped the gun on the announcement initially.
The press announcement of the closing concert, which was just released this morning, doesn't give many details of anything special that will happen. Other than "Ozzy will unleash a thunderous spectacle of rock to put the Lich King in his place and cement his own title as the one true Prince of Darkness." Morhaime goes on to say that "Many of us at Blizzard are lifelong fans of Ozzy's music, and we're thrilled that he'll be performing for all of our BlizzCon attendees..."
So there you have it. Rumor confirmed! I wonder what else might be announced... WoW.com will let you know!
Edit: Check out the Ozzy splash at the World of Warcraft Homepage.
BlizzCon 2009 is coming up on August 21st and 22nd! We've got all the latest news and information. At BlizzCon you can play the latest games, meet your guildmates, and ask the developers your questions. Plus, there's some great looking costumes.