Blizzard kicked off the week with a massive wall of text concerning the revamp of item stats that will be arriving in Cataclysm. Though I suppose that these details might be so fundamental that they're basically carved in stone at this point, usually this kind of information dump happens only very close to public testing of the changes in question. Historically, that would mean the Cataclysm alpha test, but there may be a big reason for Blizzard NOT to hold an external alpha test of this expansion.
Wrath's alpha non-disclosure agreement was basically a joke. Blizzard made the client publicly available to people who weren't in the alpha test, and all of the datamined changes promptly ended up on a wiki hosted by someone willing to thumb their noses at a Blizzard cease and desist letter. WoW Insider reposted the info, got hit with a cease and desist, and complied by replacing the posts in question with repeated direct links to the wiki, accompanied with "we can't post this ourselves but..." taunts to the lawyers. Blizzard could have stemmed much of that tide by password protecting the client download, but there probably still would have been some more conventional leaks to be had.
The problem is that nowadays we're in an era where hackers are taking out google ads against every WoW related search term imaginable, often with typo'ed URL's that direct inattentive players to keylogger sites. If Blizzard goes ahead with an alpha NDA, literally thousands of accounts will likely be compromised by the top search results for "cataclysm alpha leaks".
Realistically, is the testing feedback that Blizzard gets from the "friends and family" pool really worth all that? My guess is that it is not - if Blizzard did an external alpha of Starcraft II, I certainly didn't hear about it, so I don't see why they'd find a Cataclysm alpha any more necessary.
P.S. Bonus Cataclysm Beta Tea Leaf:
The Icecrown Citadel raid has begun nerfing itself as of today, with a 5% buff to player hp, damage, and healing. All of the encounters other than the heroic 25-man Lich King (with his 100 million hp) were beaten by the top guilds in the world without this buff, so one might conclude that Blizzard wants more players to complete the content sooner rather than later - something that would not be in their longterm interest if Cataclysm were somehow delayed and didn't arrive for a year.
The buff scales to 30% - if it increases by 5% per month, it would hit that cap in August. WoW expansion betas tend to run about four months, so the Cataclysm beta could start as late as early May and still make a September launch.