Apparently Intel changed the TIM on the Ivy Bridge CPU's. They used some thermal paste instead of the solder they used on SB, which in turn increased heat significantly reducing overclocking headroom.
IB chips ARE faster clock for clock, however, but it's not worth purchasing if you're an avid overclocker due to its heat.
Some people have gotten lucky, however, and have reached over 4.8Ghz with their IB chips.