It's a Channel 4 poll and Peter Kellner offers a full analysis here.
The key finding is that Cameron is much more trusted to look after the future. Brown's diminishing advantage refers to the current crisis. As time passes - all other things being equal - the Tory advantage should grow.
I wonder if the new tactic employed by Brown at PMQs reflects Labour's private research: "by almost two-to-one," YouGov found "voters agree that Cameron is "talking down the economy for political reasons and risks making things worse"." Labour activists have tried this tactic before.