It still interferes with itself, and that interference affects the pattern of detections. It's as if the photon were a wave right up until the moment of detection, at which points it's forced to “particalize” and pick a spot to be located at — but it's the amplitude of the wave it was just before detection that determines where on the detection screen the photon is likely to show up. If you send many photons through one at a time, the detections (each just a point on the screen) will fill out the expected double slit pattern.
As the other comments have already mentioned, it interferes with itself, so you still observe the same interference patterns [0] [1]. Which admittedly seems impossible at first, but so does the rest of quantum physics.
I've always wondered what degree of confidence exists amongst the cogniscenti that a single photon event happened. I tend to think the criteria of measurement here would suggest the most likely outcome was a shitload more than 1 photon, and that all the "but we measured we can see one only" measurements are themselvs hedged by a bunch of belief.
That said, I do like the single photon experiment, when it's more than a thought experiment.
It's a wave of probability, that interferes through the slits and then collapses into a probability of one somewhere along the wavefront at the point of detection. Whatever that means :-)
