At the start of the page I was really curious as to what image sensor they'd be using. Then I felt like an idiot.
Using a digital sensor would have several advantages in prototyping - you get a faster iteration loop of 'build camera, take picture, evaluate result', you can actually check for light-leaks in real time without removing film, and so on.
The only problem is that the sensor is probably tiny and any old lenses you might want to play with would have a massive telephoto effect, making focusing very difficult.