For a long time now Intel has locked the multiplier of its processors (except certain expensive models sold for that purpose) in order to block overclocking.  It is the only way they could continuue to sell high-frequency processors at a much higher prices than the slower ones since usually they are equally able to reach the same frequency.

To get around these limitations, enthusiasts have turned to adjusting the frequency of the motherboard's system bus.  The frequency of the motherboard is a multiplication of the frequency of the system bus by the value of the multiplier of the processor.

With it's next architecture, called Sany Bridge, Intel has combined the system.  There is now only one primary frequency for all the components.  Thus, changing the system bus frequency will overclock not just the memory but also the PCI Express bus, the USB controller, the SATA controller, etc.  With this situation, the smallest of overclock could ruin the stability of the machine or leave parts incapable of communicating.  Of course, Intel will still sell very expensive CPUs with unlocked multipliers.  It will be interesting to see if PC motherboard manufacturers will be able to work around these limitations, which is not just a sport for them but a business necessity.