Regardless of whether it's true (I happen to think it is, but I'm sure it will be argued), it's a very potent weapon for the eventual Democratic nominee. An argument that boils down to "Your War is why Americans are losing their jobs" is wildly over-simplistic, but likely to hit home quite effectively. McCain has put his entire reputation on the war, and while he may be able to sway people on the moral argument, I suspect that winning the pragmatic one is going to be a lot harder. The more people think about this link, and see McCain's justifications for the War, the crankier they're likely to get. Even Clinton can argue that she's been trying to disengage for a good while now, and that that might have spared the country the worst of the economic impact.
(Of course, this assumes that the economy is still in recession in November. I suspect that it will at least be perceived that way, even if a turnaround has started by then -- it takes time to shift the public perception, and I don't expect this particular setback to be either mild or quick...)