Accelerometer data can be used in a variety of ways. The rotation modes of the iPhone, for example, require that a certain voltage is reached and sustained for a period of time by the changing resistance inside of the accelerometer. A fast or slow rotation should both produce the same result - the voltages summing over time, in effect. Then, the iPhone does the rotation transition, similar to a Keynote transition, and all of sudden it's like you changed screen orientations.
I say this YouTube demo is real. All you have to do is stream the accelerometer data live and relate the data to a graphical display transition, in this case rolling marbles or that one game thing. It's a relatively simple amount of code to write, even in, say, LabView or MATLAB. I think the harder hack is getting it to actually run inside of the iPhone.