Currently, high-end mobile CPUs are clocked in the 800-1500MHz range, depending on the manufacturer and core count. The Samsung-built Apple A5X in the iPad 2 and 3, for example, is clocked at 1GHz, while NVIDIA's Tegra 3, which utilises ARM's Cortex-A9, is set to 1.3GHz in the ASUS Transformer (1.4GHz in single-core mode). I don't think anyone's complaining about the performance of these chips, though faster is always better as long as power and heat aren't increased noticeable. Makes you wonder what we'd do then with the 3.1GHz Cortex-A9 ARM's managed to put together.
With the help of TSMC 28nm "HPM" process, ARM has tweaked dual-core Cortex-A9 to run at 3.1GHz "under typical conditions", according to DigiTimes. However, comments from TSMC state that the processor would likely operate in the "1.5GHz to 2.0GHz" range for mobile devices, while the almighty 3GHz+ reserved for "high-performance uses". It beats the pants off chips constructed at TSMC's 40nm process, which is used for ARM's current version of the Cortex-A9. For reference, the A5X is built using Samsung's 45nm process.
High speeds are great, but one has to wonder about power consumption and heat generation. Dropping to a smaller manufacturing process will help with these, but I imagine ARM and TSMC will continue to play around with the design until it's a realistic option for tablets and phones.
The other thing to point out is that megahertz don't mean everything, as Intel's NetBurst architecture aptly demonstrated. Still, you can get a lot done at 3.1GHz -- heck, a dual-core 2.0GHz chip should service our mobile needs for many years to come.