If you recall, when the Cube came out it went against the new division of the Apple product line up:
One of the first things that Jobs did when he came back to Apple was clean up the product line which consisted of a thousand variants of machines (The myriad of PowerMac 5500/6100/8100/5560/6580/ etc. etc.) and reduce it down to 4 products: Pro, Pro Portable, Consumer, Consumer Portable.
The Cube went against this as it was neither Pro, nor Pro Portable and was too powerful to be Consumer.
At the time, there were only the blue and white G3, the PowerBook Wallstreet, the Bondi iMac, and the clamshell iBooks. The Cube was in between each of these categories.
With the categories now a firmly established staple of Apple now, it seems likely that the Cube would have been better suited to the Consumer range of products. After all, it has limited internal expandability in the same way that the 17" iMac does. What made it Pro when it came out was that it was a G4, and that sort of power was "reserved" for professionals.
Now that we're seeing the iBook and iMac line using the G4 chip, and pro moving to G5, the "old" cube would only need to be painted white to fit in perfectly to the lineup.
The question remains: how will they ever top the sheer genius of the connecting arm for the 17" screen?