You'll see plenty of stories, rumors, made-up things, halfway-true things, and perhaps even completely true things today about Juan Mata, Manchester United, and the Tooth Fairy. Believe them; don't believe them - it's your prerogative.
However, as far as I can tell, the one source ("source") that everybody not touting some hilarious player swap story keeps falling back on is still just The Guardian's claim from Tuesday that Chelsea have agreed on a £40m transfer, in principle (whatever that means):
Chelsea and Manchester United have provisionally agreed a £40m fee for Juan Mata, though the champions are yet to make a formal bid and any deal to take the 25-year-old to Old Trafford remains dependent on him personally informing José Mourinho of his desire to move.
Negotiations between officials from the two clubs are at an advanced stage, with the transfer said to be "almost complete" by sources close to the discussions.
-source: Guardian, 2012.I.21
Please note key words like "yet to make a formal bid" and "sources close to the discussion" and something about a transfer request. Strong stuff, said no one ever. Or perhaps you said it yourself, I don't know. It's tough to keep track at this point.
Props to Sky Sports for trying to up the ante and cash-in on the frenzy with their tweet, but it turns out they revealed nothing new.
And now you know. As you were then.