Got a Mac Mini years ago, Core 2 Duo and smaller than an ASRock, bloody expensive though ;( But thats the price you pay for Apple.
I think theres two very valid points in this debate, A prebuilt one has the advantage of smaller size but less customization, but a build your one allows more customization and perhaps even a better price point as you can buy what you want.
The size difference between a Mini-ATX case and a prebuilt is different (prebuilt usually being smaller), but both are still massively smaller than a normal PC case so unless the size savings are important to you in that scale a prebuilt one may not be the best solution. Its all down to what people want, I myself would most likely go with a prebuilt one, but if I had the money and time I would like to build a really powerful small PC.
I also assume power wise, the biggest drain on power is the motherboard, cpu and gpu. Now having an Ion, Atom and mini motherboard which is in both the prebuilt and custom made should make both around a similar power level I assume, but I am open to corrections if people wish to do so as I am not an expert in this field.
The definition of nettop however is pointless, espically quoting wikipedia. Dell classed there 12" and 14" laptops as netbooks, now in my mind I wouldnt class them as such. The definition isnt set in stone and it constantly changes with the market. Now why dont you all call down and keep talking about the options.