TechPowerUp reports overclockers discovered that there's thermal paste between the CPU die and the internal heat spreader (IHS) in Intel's Ivy Bridge processors. It's speculated that this may be part of the reason why Ivy Bridge runs hotter, as previous-generation processors used flux-less solder with a higher heat conductivity.
In comparison, Intel used flux-less solder to bind the IHS to the die on previous-generation Sandy Bridge Core processors in the LGA1155 package. Attempting to remove IHS off a chip with flux-less solder won't end well, as it could rip the die off the package. On the other hand, the idea behind use of flux-less solder in CPU packages is to improve heat transfer between the die and the IHS. Using thermal paste to do the job results in slightly inferior heat transfer, but removing IHS is safer. One can be sure that making it safe for IHS removal couldn't have been the issue behind switching back to conventional thermal paste, as everything under the IHS isn't user-serviceable anyway, and off limits for them.