This is a public service announcement from all of us here at WoW Insider: there is currently no beta running for Wrath of the Lich King. This means that if you receive an invitation that claims to be for a Wrath beta, it's definitely a scam and you should under no circumstances click the links therein. If there were an actual Wrath beta, you would surely hear about it here and/or on the official website. If you have signed up for any alleged beta test, it would be a good idea to change your account password as soon as possible; you can't be too safe these days.
Oh, and while we're on the subject of misinformation about Wrath, release dates on retailers' web pages mean nothing. They have to make a date up in order to enter the product into their system most of the time, but unless they're Blizzard, they're just guessing, and your guess is probably as good as theirs. Now have a nice weekend, and don't sign up for any dodgy beta tests.