OK, this is a mad blue-sky idea - feel free to shoot it down (but please explain with facts and numbers, rather than just insult me for coming up with an idiotic idea)
I remember back when I was crunching for Seti@Home. I'm a Mac and OS X enthusiast (note that I'm *not* an Apple fanboy - some of their business practices and products piss me off) and have some fairly serious Mac gear. The calculations required for Seti are similar embarrassingly-parallel computations that lend themselves to distributed computing. At the time, before GPGPU code became the norm, CPU crunching was the competition, and I once got to the top of the tree (recent average credit, #1 worldwide) with my mad Quad G5 PowerMac.
The Quad G5 PowerMac was an insane machine - stupidly expensive, needed liquid cooling for its CPUs, and when crunching work units, the northbridge ran at a nice constant 110˚C. That is *not* a typo, nor do I work in Fahrenheit. Eventually the G5's liquid cooling system started to fail, so I moaned a bit to Apple about only getting a year out of a £4,500 machine, and they gave me an 8-core Xeon Mac Pro for the price of a Mac Mini, which was nice of them (but a different story)...
The reason why the Mac G5s were so highly-represented on the Seti leaderboard (against Opteron servers and similar Intel server kit) was due to the PowerPC chipset, and specifically the vector processing instructions (especially the 'vector permute' operation). This gave a huge shortcut, allowing the Seti processing (IIRC, lots of FFTs) to do the same work in many fewer clock cycles.
GPUs operate on similar principles and are clearly the fastest hashers for Bitcoin mining. But they use a LOT of power, and the cost of electricity is rapidly outstripping mining profit. Hence miners are looking to lower-power solutions.
The FPGA approach is being actively worked on by a bunch of enthusiasts, but the FPGA boards themselves are expensive, they're not simply 'plug and play', and there's the question of whether one can legally make money mining when very expensive software licences are required to load the code onto the FPGA.
I was always impressed with the optimisations one could achieve with the PowerPC instruction set and the Altivec instructions - PowerPC has loads of registers, and the fancy 'vector permute' instruction. There's no way an old Apple G4 CPU would be a fast hash engine for BTC mining, but with massive optimisation, and new CPUs (the G4 is still produced for routers and other embedded kit - low power consumption is a priority), would a big parallel array of G4 boards be anywhere near usable for mining?
TL;DR: the old PowerPC chips Apple used (G4 onwards) had vector processing instructions. Could highly-optimised code run well for BTC mining, if one focuses on hash per watt? FPGAs will win but they have a monstrous upfront cost, whereas a load of dual-core G4 and G5 Macs can be acquired relatively cheaply. Making a bare-bones machine would consume even less power. Does anyone know - and has anyone tried writing a highly-optimised hashing function for Altivec?