The beta test is beginning to ramp up. Tonight we have word that the beta downloader is in action and delivering data to those that have access to it (via WotLK Wiki). This is an obvious indicator that Blizzard has switched over to the beta testing phase, and that they are getting ready to send out invites.
Additionally the beta page at beta.worldofwarcraft.com now shows beta information and activity. If you do get a beta key, you can use this page to sign up and gain access to the client files. It would be a good time to start keeping and eye out for a real beta invite, and let us know if you get one if you're one of the first lucky few. However at the same time, please be very careful not to fall for any beta scams.
WoW Insider will keep you posted throughout the evening on this breaking news.
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