I think the problem stems from people comparing Dion's numbers to Martin's on election night and Harper's to Harper on election night. In reality, the dynamics have completely shifted since then. We're not in a campaign and the Liberals are now the opposition party while the Tories are in power. The sitting PM always does well in these polls because Canadians are more exposed to him than the opposition leader so they're more aware of his vision and naturally more trusting. And, of course, they'll see him as a better leader because he...is...their...leader. Here's an SES poll from 2005, showing that everyone thought of Martin as a better PM than Harper...mainly because he was the PM.
So the real comparison should be between the positions, not the parties. Ideally, from some time like September 2005 when people weren't in "election fever". The oldest poll I could find asking the "leadership questions" was from Nov 30/Dec 1, 2005 (I averaged the two), right at the dawn of the last campaign.
Here's the comparison:
Dion 2007/Harper 2005
In other words, Dion is viewed exactly the same way as Stephen Harper was before the last election. But before Liberals rejoice, here's the PM comparison:
PM Harper 2007/PM Martin 2005
So while Dion is as strong as Harper was in 2005, Harper isn't the crippled PM that Martin was in November 2006. And that figures, given that this poll was done with the Gomery Report still fresh in the minds of Canadians (Gilles Ducceppe was the most trusted of all leaders!).
So my take home message would be that there's no need for Liberals to panic or for Dion to re brand himself, but that Harper should not be underestimated. With that in mind, rushing into a spring vote would be a foolish move on the part of the Liberal Party in my opinion because, despite some mis-steps, Harper hasn't been crippled enough yet to move in for the kill.