This reaks of phishing scam to me.
The minute you type your account details in to whatever site this links you to the scammers will have it, and the next time you login to your WoW account you'll prolly either find your characters completely stripped or they'll have changed your password and you won't be able to login at all. Not to mention all your personal details.
Since WotLK was only released a couple of days ago, Blizz aren't going to be well in to the development of another expansion pack already (i wouldn't have thought?). And even if they were, it certainly wouldn't be at the stage to invite players surely, could be wrong but i wouldn't risk it personally. You could always email blizz themselves and ask em just incase.
[Edit]
Did the email look like this?
http://forums.worldofwarcraft.com/threa ... 1040&sid=1
(Quick way of checking links in e-mails when you're unsure btw.
eg.
http://www.worldofwarcraft.com (click that link). Tis very easy to disguise a link like that to make you think it's taking you somewhere "official" but really somewhere different. If you just right click the link there and click "copy link location" (in firefox) or "copy shortcut" (in IE) and then paste that into the address bar, you can see it's really taking you to google, not WoW.com. Do the same for the link you got for your alpha invite and see where it really leads (no need to actually go there).
You get the same phishing emails for things like ebay as well that want you to login, some of them can look highly convincing as well but you can use that method to double check whether they're real or not usually. I once fell for one of those a long time ago, thankfully i realized what it was just seconds after i hit submit and managed to change my login info though, could have been much much worse.
I have been invited to the ALPHA for "the Emerald Dream," but at the mo' I can't afford the WotLK expansion, which I apparently have to have to play in the Alpha for ED.
