"Isn't that like every digital piano? Using a powerful CPU and a good sample is exactly the combination for getting the most realistic sound (the action, one can debate over)."
Not exactly. Digital pianos (at least the ones on the market today, use purpose built operating systems and effects processors and DSPs. Their CPUs are not Wintel (desktop application) type devices. That's not to say that using a general purpose computing environment is bad (today) but wasn't really practicle for the last couple of decades because of (performance, reliability, licensing, software updates and compatibility, etc.)
Clearly the dedicated computing infrastructure of designs of digital pianos were vastly superior to general purpose computers. Just compare the quality of of tone and timbre from boards like the Yamaha CLP-170, Roland HP-107, GEM Promega3, and Kawai 9500 with even the best multi-gigabyte software sample.
I think it is also obvious that times are changing (dramatically). General purpose computing has become so powerful and cheap, that even features such as real-time convolution reverberation are possible (GigaPulse). If this is possible with PCs, just imagine what will be coming out with the next 1-3 generations of purpose built digital instruments.
I got off topic. ;-)
I was simply stating that the Postpiano was a general purpose computing solution that many people put together on their own, while most people (well at least me)would think of a digital piano as a purpose built device.
Rodney