One them doesn't, but if you cover a board in themARM simply doesn't have the horsepower to compete with an Intel mobile or desktop CPU. Great for low power devices but not in the same ballpark I'm afraid.
Well, still not ideal for professional apps, better for web-servers and other high-volume but relatively low demand (individually) processes.
But that said, if Apple's vision of performance processing on GPUs pans out, then ARMs may actually be just fine as CPUs; I don't really see it happening personally, but if you were building a high-end GPU computer cluster then ARMs aren't a bad choice if all they need to do is hand out work for the GPUs to do and move the results somewhere.