Emerging Processor Lines
We already see three major lines of processors targeting mobile, desktop, and server markets. This trend is likely to continue and result in appearance of even more processor lines optimized not only for various segments but for various applications or intended uses as well. For instance, for application servers we may see Intel and AMD delivering vastly multi-core CPU with good integer capacity and dedicated encryption/decryption hardware in the vane of Sun's UltraSparc, while in mobile market we may see stripped-down extra-low-power CPUs that ensure very long batter life, perhaps with finer frequency scaling similar to what coarse-grained AMD's PowerNow! technology does now. There certainly seems to be a room for lower-performance CPUs for ultra-mobile computers since most of them are used for reading, browsing and other simple tasks that do not require much of CPU power (specific tasks such as multimedia encoding/decoding and 3D graphics are already partially offloaded to dedicated hardware and are likely to be even more confined to specialized chips in the future).
So focusing on mobile CPU market it is clear that power efficiency and that not only of CPU but of the entire system is likely to be much more important than raw processor speed. After all most mobile users are not likely to exploit potential CPU performance to the fullest extent unless we throw at them really bad code. Almost commodity pricing on computational power today is such that consumers can afford buying more and more specialized hardware that is better suited for a particular purpose thus fulfilling Bill Gates' vision of computers in every pocket. This is in fact already happening as we all are grabbing iPods, cell phones, PDAs, and BlackBerry devices to complement our laptops and desktop PCs. No more one-size fits all. This is the most certain prediction that one can make about future CPUs. We shall see more and more specialized models and not necessarily more powerful ones. Thus as far as mobile market is concerned we might see CPUs with more finely grained frequency control that responds to idle time, variable rotation rate hard drives and possibly stripped out of some advanced features such as enhanced multimedia processing instructions in favor of dedicated hardware performing the latter tasks.
In fact AMD is already making some steps in this direction with its upcoming 4x4 platform and open specification enabling 3rd party co-processor design. In the long term it makes little sense to burden CPU with DVD playback or SSL encryption. These and similar tasks should and with time will be handled completely by dedicated hardware that is going to be far more efficient (power and performance-wise) than CPU. Further variety of coprocessors will allow enhanced physics and environmental effect experience for gaming enthusiasts and improved performance for scientific/multimedia applications. Thus the role of CPU is likely to diminish with time living little reason for further clock-speed improvement.
Frankly the role of CPU as a jack of all trades started to wane with the advent of GPUs. 3D graphics was the most compelling reason to boost CPU power. Now PCs typically have a dedicated processor (or two in the case of AMD's 4x4 platform) that is far better suited for the task. Similarly most music/multimedia hardware relies on its own expansion boards outfitted with custom logic/DSP processors (take ProTools or Creamware products, for example). And with time we are likely to end up with a motherboard design that would contain numerous specialized chips or co-processors designed with a single task in mind. So in this respect we are back to the single-purpose computing we have started with, although such return is a mere new loop in the spiral.
Ironically, return to special-purpose computing results in further relaxing of requirements for higher processor performance: special-purpose code is usually better optimized and thus can perform equally well on much slower CPUs. In reality most hand-held devices are powered with few hundred MHz CPUs that are capable of providing similar experience (save for small screen and tight keypad) we have with our gigahertz-fast desktop PCs. Similarly specially-designed DSPs are far better for MPEG playback or sound processing than general-purpose CPU that can do the same running at high GHz.
In other words, what is likely to happen is that CPU frequency increases will become very modest in the near future. As hardware manufacturers compete for the markets we are likely to see less and less general-purpose and more and more specialized hardware for various purposes. Perhaps in 10 years today's Athlon and Xeon CPUs would seem like dinosaurs, hot, big, and less than bright, with the role of CPU in the computer reduced from the do-it-all-yourself to coordinate-the-work-of-others.