Im not an expert or anything but i know eventualy it will be voltage, temp, instablility, or a combo of them. Ive heard of pens bridging at a certen voltage also (dont quote me on anything, just my $.02)
Temperature would rarely become an issue with most enthusiast overclockers as they'll previously take measures to prevent it from creating a ceiling
Voltage would be the main limiting factor considering in theory you should be able to clock a core much higher assuming the transistors could handle those sorts of environments in a 24/7 setting
Of course if you wanna get really technical you could assume your BIOS settings could limit your overclock assuming you are able to reach your maximum multiplier and FSB settings and feel you can still go further
oh interesting, so presumably you may be able to alter the mobo so its capable of delivering more voltage, and then some how edit the bios so you could really increse the fsb
basically there are many controlling factors that limit overclocking. the ultimate factor would be the voltage limit of the processor or when electrical crosstalk becomes a problem