[citation][nom]g00ey[/nom]It would be interesting to know why ARM has been so successful in portable devices and not MIPS or DEC Alpha, or PowerPC.[/citation]
It seems that ARM has been so successful in that area because they have very little competition in the ultra-low-power processor market. ARM is the only architecture that doesn't have (nor ever really had) processors that competed at the high-end. Early on, they decided to focus exclusively on the (less interesting imo) low-power market, and so it's little surprise they excel in that area.
However, just because they have the best perf/watt at the low-end doesn't mean that will translate right on over to the mid/high-end. Due to the non-linear relationship between power and performance, lower-power, lower-performing processors almost always have better perf/watt than their better performing peers, regardless of architecture. Likewise, perf/watt only tells part of the story; absolute performance still matters! Even in this "age of parallel computation", serial performance is still important in the vast majority of tasks. So even if you were able to get more aggregate performance from a ton of ARM cores than from a lesser number of x86 or POWER cores in the same power envelope, that doesn't necessarily make it more desirable. To use an extreme example, would you rather have 4096 (hypothetical super-ultra-low-power) 486s or 4 Core i7 cores at your disposal?
To rephrase the original statement: it would be interesting to know why PowerPC has been so successful in the high-performance embedded market. Obviously that's what IBM (and to a certain extent Freescale) decided to focus on. ARMs highest performance cores of today still can't compare to the PowerPC-based processors used in the Xbox 360 and PS3 of half-decade ago. It's all a matter of focus. I actually consider the game console embedded market to be the most interesting of all, because performance, power, and price all have to be balanced. It will be very telling to see which architectures wind up in the next generation of consoles.