IIRC, x86 has more instruction optimization points than ARM does (think mmx, sse2, ssse3, vmx, etc). This means that there are more areas for compilers to optimize on and Intel has lots of resources to throw at compiler optimizing for GCC, and the like. This is one of the reasons atoms run stuff faster at the same clock than ARM. Also, Intel has very evolved manufacturing techniques. ARM Holdings has zero manufacturing facilities. They just design the chips and they are a not a very large company. Furthermore, TI, Qualcomm and the like don't even come close to Intel when it comes to these techniques.
Also, with regard to the AMD Bobcat... I hadn't heard that AMD was even targeting the smartphone/ultra-mobile sphere. Bobcat sounds like a netbook/tablet product. They need sub 1W Active power usage to come close to current ARM and soon Medfield product lines. However, the integrated GPU in the bobcat does sound as appealing as the Tegra lineup. Could be really interesting. But along with the Desktop lineup, AMD is playing catchup on a very large scale.