Here's a PC with a 1-inch-thick case by Trident, and it's so quiet and well-hidden that the company calls it "invisible." You don't see it? It's right there, attached to the back of that monitor pictured above. It has the industry-standard VESA mounting holes on either side, so you install it in between a mounting arm and the monitor itself. Its fanless design keeps things super quiet, and you can also put a 40GB hard disk inside.
This PC is not exactly a powerhouse—it has a relatively lame 1.5GHz Via Eden processor on board and is designed for digital signage and retail displays. However, it's just a hint of how PCs will be embedded and invisible in the years to come. Soon we'll be knee-deep in ubiquitous computing without even knowing it, where the PCs will be everywhere and appear to be nowhere.