Taken from MMO-Champion:
Mists of Pandaria Internal Alpha Started
A lot of people have been asking me when we should expect a MoP beta or more updates on the game, and the answer to that question is simple: soon. All the recent events indicate that the internal alpha testing already started:
The old WoW2 Battle.net accounts used for the Cataclysm Beta have finally been removed from some players Battle.net pages.
The recent update of the official talent calculator had multiple spells tagged as "NYI" (Not Yet Implemented). This is what we see in Alpha/Beta stages of the game to indicate that a spell doesn't work in-game yet.
Community Managers have posted actual screenshots of new spells tooltips in-game. It means they have a server they can easily connect to for that, and it's most likely the biggest proof.
Now what does it mean for players? First, let's be clear, this is the internal alpha and it is different from the Friend & Family Alpha where non-Blizzard employees can play as long as they know someone in the company. This one would be restricted to employees only and Blizzard executives are probably watching anything happening there very closely.
However, it definitely means that if everything goes well we should see a Friend & Family Alpha or a Beta within one or two month, and the Mists of Pandaria press tour on March 19 is most likely a way to prepare fansites to that. For Cataclysm, the press tour happened about 2 weeks before the start of the closed beta.