Dubner, on the Freakonomics blog, asks "Does it really matter who is president?" http://freakonomics.blogs.nytimes.com/2007/08/13/how-much-does-the-president-really-matter/#comment-99759
It's a plausible argument, but comes at a curious time.
Starting a stupid war kills hundreds of thousands of people [the majority being Iraqi civilians] who are dead forever. It's quite likely that Bush is one of the few people who would have made that decision. Remember all the Bush supporters who pointed out how Bush was a man of action, whereas Clinton has limited himself to bombing, boycotts, and diplomacy.
To me, this post sounds like what a lot of former Bush supporters are saying now to excuse their past support of W. There are several variants: "Bush really wan't a conservative, so his failure doesn't tarnish conservatives." is one common one.
This one is "it doesn't matter who was president -- all this bad s*** would have happened no matter who was president."
Face it, those who voted for Bush: the guy is/was a disaster, and you are responsible for putting him there. It's an ugly truth, but it's truth.