Mobile architectures are typically only a tock away from desktop architectures, who aren't really guzzling energy with these optimizations (the deep pentium 4 pipelines are behind us!). So if technology marches forward, there isn't some mobile version that is frozen in time just because of energy efficiency (indeed, good branch prediction saves energy also). There are no 386 or 486 CPUs to care about now, while P5s are limited to that Phi HPC product.