Its not long since the Samsung Galaxy SII made it on the shelves. The dual-core smartphone already very fast, got even faster after a few days since it went on sale, and its already being overclocked to a stable (tested) 1.5Ghz.
The OC went even further to 2Ghz but became very unstable because of the high voltage and high temperature, and a little less unstable at 1.8Ghz.
According to coolbho3000 developer of SetCPU and member of XDA-Developers Forums:
I have successfully overclocked the new Samsung Galaxy S II to 1.504GHz. This speed is enough to allow it to achieve really high benchmark scores like over 4000 points in Quadrant! 1.5GHz is a 25% overclock and makes this fast phone even faster and snappier in day to day usage.
I haven’t gone crazy on the voltages – 1.504GHz is stable at 1375mV on my device. Depending on your device, there may be even more headroom. I’ve gotten speeds of up to 2GHz to boot with instability (my unstable 1.8GHz Linpack run is currently in the Linpack top 10), but at very high voltages and temperatures. Leave it up to other developers to give you higher/more dangerous frequencies and voltages. If you stress the CPU a ton the phone will become hot to touch (though it does to a certain extent even at stock frequencies – there is not much thermal insulation between the processor and your hands in such a thin phone).