Although it will likely be a long time before running chips at this speed in a mobile device will be worthwhile, the mobile chipset foundry, TSMC, pushed a 28nm ARM Cortex-A9 dual-core chip to 3.1GHz, just because they could. Obviously, heat and battery consumption make this CPU speed impractical in an actual device, nevertheless, it is still impressive that they are already pushing clock speeds that far. Eventually, this will evolve into something useful as the die on these mobile chips continues to shrink. The "holy grail" of mobile computing will be a chip that operates at low power, produces almost no heat, yet still cranks out fast calculations at a high clock speed coupled with multiple cores to make things more efficient.
We may not see actual devices with clock speeds above 2GHz for quite a while, but it's good that they can do it already, because it points to a bright future.