As the Sandybridge is fairly new it would be impossible to say if it would effect life expectancy of the processor, however I am sure that PCSpecialist have tested overclocked Sandybridge processors rigerously with Prime95, etc to make sure they're stable.
As the life of a CPU is generally 15 years, halving that by overclocking will still mean that the CPU lasts a lot longer than the typical PC before it gets replaced.
That said the 2600k is a great clocker and many people are getting above 4 and even more on stock cooling, so there is very little risk of long term damage here.
And again with that said, im talking about enthusiasts who know what they are doing, going into the BIOS and attempting an overclock without extensive research and a deep understanding will usually shorten the life of the CPU to around five minutes.