One thing that's come out in this discussion is that the word "great" has varied meanings. So I'm going to start by defining my terms:

A great president is one whose accomplishments result in a stronger, more prosperous country, with better diplomatic prospects, a more secure peace, happier and more prosperous citizens, better educated people, and generally better lives for most Americans.

A bad president is one whose accomplishments, or failures, result in a weaker, less prosperous country, less respected around the world, less secure in its peace, with discontented and impoverished and more poorly-educated people, and generally worse lives for most Americans.

By that standard, I think the answer is pretty obvious. If in the long run Bush's accomplishments fall into the second category, it will be because his dismal failures have pushed us so deeply into the first that public outrage provokes some real reform that has been needed for decades. In that sense, he will be "great" in the same way as James Buchanan, whose fumbling politics brought on secession and the Civil War, or Calvin Coolidge, whose short-sighted economic policies led to the Great Depression. Generally, however, we apply the "great" level in those eras to Abraham Lincoln and Franklin D. Roosevelt, who cleaned up the respective messes.

The same will, no doubt, be the case this time around.