Fascinating! , and yet nobody has mentioned the core problem of heat.
Heat is what places constraints upon the upper limit of the nano scale transistors switching, clocking a computer above 4ghz can be done, but not for very long ! a few tenths of a second and the substrate becomes molten slag. Of course as Nigel pointed out silicon is not the only semi conductor, now silicon has a fairly low melting point and most computer chips start to show serious distress if allowed to exceed a mere 70 degees centigrade. The solution .. a semi conductor more tolerent to heat is required, well one does exist. Crystals of pure carbon are at present the focus of much research, though quite how an average home user is going to cool a motherboard running at over a 1000 degrees centigrade will present a challenge all by it'self.
Tinkering with the architechture gets around some of the problems of silicon, IBM's approach of four processors in a single package is being watched with some interest. Wether Microsoft is going to embrace this distributed computing is open to much speculation with many large manufatures and server operators already running Linux.
Of course technology might make a U turn and ditch silcon transistors as an engineering dead end , returning to vacuum thermionic devices wich are heat tolerent and resistant to damage from electromagnetic pulses.
Whilst I doubt the glowing tubes of our grandfathers will make a comeback, the lessons learned from etching silicon allows for the devices to made immensely small.