He didn't do it on MLK day, like everyone thought he would, but it's supposed to be forthcoming.
He's on Oprah Wednesday, as Geraghty notes. It seems to me that announcing there would do wonders for his popularity, fluffy-factor, hipster air, and get him tons of earned media-- none of which he needs, although I guess you could make the argument that he can't get too much of a good thing.
What he does needs is some seriousness points, some Senatorial bonafides, and Oprah won't get him those. Still, if he announces on Oprah, his announcement will have 40 clips on YouTube and be featured on every pop culture blog in the blogosphere-- all of which are populated with huge numbers of the young, apathetic non-voters most likely to be swayed by his empty charisma.
If, as Dick Morris says, Obama's and Hillary's strengths are that they can create new voters instead of relying on simply getting out the old ones, then this move by Obama could pay off. He increases the coolness-gap between him and Hillary, gets a softball forum in which to announce, and signals his leaving of the old rules behind. It's so rock 'n' roll.
But how will it play in D.C.? An announcement on Oprah? It will not please the elders. That's a lot of fluff to slough before he can get his serious on. And, he's gotta do that at some point.