Now before you go and tell your friends and family what really is important to you (because lets be honest, WotLK beta > 4th of July, right?) you need to consider that this is nothing more than a big ol' rumor.
This information came from Gnorog when he was at the World Wide Invitational in Paris this past weekend. I know that Gnorog exists and is a German community manager, but I can't confirm that he was at the WWI (although I'm sure he would have been). We get a good amount of beta and release rumors every day here at WoW Insider, and half the time they refer to people that aren't even real. So this rumor gets +10 for referring to a real person.
As always Blizzard is being very tight lipped on this. Bornakk recently locked a thread discussing this date in the WotLK forums. He gave the standard line "We will provide further information on our forums when details on a beta testing phase are ready. For now we have no date to provide regarding this." There have also been a few threads
that have appeared on the German WoW forums, but they have not received any comment from Gnorog.
We'll have to wait and see what happens. Check back on WoW Insider throughout the day on July 3rd to see if the beta does drop. That, or you can just listen for my scream of joy when I get my beta key.