The Apple Cube was a fanless 400MHz G4 three years ago.
This is not 100% correct. The Cube was a fanless 450MHz G4 :-)There was also a version with 500Mhz (also fanless), which was said to cause heat problems. I can't confirm this, my 500Mhz cube is in use daily and I have had no problems so far.
The cube is my fastest machine, because I refuse to buy new computers with fans. I hope Apple will release a new fanless computer soon. I might buy it ;-)
andi