Update:

> I doubt the A6 will clock higher than 1.2 GHz in the iPhone.
> Maybe later in the next iPad, as the tablet form factor
> allows for a bit more power consumption.

As we know, the A6 chip in the new iPhone clocks at 1.3 GHz. The A6X chip in the new iPad announced yesterday is said to run at 1.5 GHz and thus faster than A6, as expected:

http://arstechnica.com/apple/2012/10/deducing-details-about-apples-a6x-processor/