Intel has been pushing for higher clock rates for ages now. While 4 and even 5 GHz is not entirely uncommon for those wishing to step outside Intel’s specifications and push the frequency as high as it can go, Intel has yet to allow their parts at that frequency in any supported fashion. That has recently changed with Intel’s Xeon line.
Tom’s Hardware noted from Intel’s spec sheet that Intel’s Xeon E3-1290 is clocked at 3.6Ghz with its Turbo Boost rating on single-threaded applications spiking to 4 GHz. Their original intention with their Netburst architecture from 2004 was to peak to ridiculously high frequencies but they quickly found their scalability ended below the 4 GHz line killing their plans for a 4 GHz SKU. With the Xeon architecture quite close to the higher-end Sandy Bridge parts it is possible that we might see 4 GHz in the desktop soon.