As others have pointed out, pure CPU-based mining, even with uber high-end PCs standing alone, is largely obsolete. Looking at the next step down, a high-end Android phone with 1GHz ARM CPU is approximately equal in computing power to a 500MHz Pentium III (give or take, but it's a fairly reasonable approximation). A gigahertz of RISC is worth a lot less than a gigahertz of CISC. The advantage of RISC is that RISC (in theory) can be scaled more easily to run at higher speeds than CISC. But when comparing a RISC to a CISC CPU at the same nominal speed, and specifically an ARM to a non-Netburst Intel-Architecture CPU, the IA32 CPU is going to stomp all over it.

OK, now that I've established that perspective, consider further that most POS hardware has -- at best -- an ARM CPU running in the neighborhood of 200MHz, and there's a LOT of hardware out there running on m68k Coldfire & Dragonball cores that are nominally ~40MHz, but roughly comparable in performance to the same 100-200MHz ARM cores (m68k is CISC... though later Coldfire variants kind of muddy the equation).

Anyway, the point is, in order to meaningfully make use of POS-type and other embedded hardware, you're absolutely going to have to treat them like army ants and implement it as a massively-distributed system whereby you have "controllers" that orchestrate the individual computing actions of the individual devices, and do it in a way where the communications overhead doesn't end up neutralizing the actual work done by each individual worker. A good analogy is to look at programming the PS3 cell architecture. In fact, I'm surprised the PS3 hasn't gotten more attention, because a mountain of PS3s would probably be the most cost-effective, Sony-subsidized computing array you could buy if a Bitcoin mining app were available for it.

Either way, any application that targets anything less than a high-end Android phone is probably a waste of time. And anything that targets Android phones that are actually being used as PHONES is an equal waste of time, because a useful bitcoin mining app would nuke most batteries within an hour or two. On the other hand, an app that's intended to make use of hundreds of OLD Android phones that are connected to chargers, networked via wi-fi, and would otherwise be sitting unloved and unused in a drawer somewhere might have interesting possibilities... especially if you have physical access to a storage room with crates and crates of such phones.