Obviously, it makes good political sense for Obama to try to talk down the state of the economy. That way, if a recovery comes about sooner than expected, he can take the credit; if things remain difficult, it preemptively establishes a scapegoat.
But what shouldn't be underestimated is the degree to which the choices Obama makes now will, in fact, shape how the economy performs in days to come. As James Pethokoukis over at US News & World Report astutely notes, Obama should be accountable for how the economy performs, given how actively he's meddling with it:
It will be his decision to forego deep and permanent new tax cuts, his decision to not extend the Bush tax cuts, his decision on how to spend the remaining $350 billion in TARP money, his decision to quasi-nationalize healthcare, his decision to push a cap-and-trade carbon emission program and his decision to spend hundreds of billions on a "green" industrial policy. It might even be his decision to try and reunionize the American laborforce.