Here in Canada, the '12 Focus is rated at 59 MPG (again, imperial gallons), so the jump is not as big as the 40 MPG rating in the US.
However, 21 MPG is still a big jump. I would consider, especially when you consider how good new diesel engines in cars are these days. But our diesel fuel is not designed to be a top seller, and therefore tends to be of lower quality than our European counterparts.
I think that if we had diesels like they do across the pond, and the resources to properly fill the tank with good diesel, I would be inclined to buy. However, thanks the American mentality (and American gas companies) I doubt we will see that kind of change in our lifetime .