We had some hints that this would happen soon and now WoW Insider has confirmed through anonymous sources that the next expansion, Wrath of the Lich King, is in closed alpha status, and that various players are being invited to check it out, under a strict NDA.
Alpha is part of the software release life cycle -- a software product in development goes from alpha status to beta status (sometimes including a "closed beta," where a limited few are invited to preview and test the software, and an "open beta,", where anyone can download and try out the beta), to "release candidates" and then "gold" (the final version, used to print the media to be available for sale). This doesn't tell us anything about the timing of the expansion's release (especially since Blizzard historically takes their time going through this cycle), but it does tell us that Wrath's content is in a playable and mostly completed form -- quests, game mechanics, and items are in, even if specific flavor text, names, and even textures are not.
We'll keep an eye, as usual, on any other news we see coming out of Blizzard, through official or unofficial channels. Wrath of the Lich King, the second expansion of World of Warcraft, is in alpha testing. The journey to Northrend has begun.
Update: A word to the wise: this is not an open beta, and any email or site that tells you to sign up for one is still scamming you. When open beta comes down, we'll hear about it on Blizzard's official site. This is just news that the next expansion has moved up a notch in its development, and that we're one step closer to an official open beta.