By filming the resulting jets of foam, and doing some Scanning Electron Microscope analysis, they've ruled out chemical interactions, and have discovered it's to do with the surface roughness of the Mentos, the sugariness of the drink and how quickly the mints sink.
These factors all affect formation of carbon-dioxide bubbles: the spikes on the Mentos aid micro-bubble formation (see mint ones on the SEM image on the left, fruit on the right); non-sugary Diet Coke works best as it's got lower surface tension; and the dense Mentos sink quickly, creating bubbles at the base of the bottle that cause spontaneous formation of other bubbles higher up. All that results in very rapid bubble-formation, and that then causes the satisfying squirt of brown goo from the bottle neck.
So now you know: science is fun. [New Scientist]