Ivy Bridge will be 22nm not 25nm. The heat produced by a chip depends more on its clock speed and efficiency than strictly its die size. Though a smaller die size allows the chip to be more efficient, this efficiency is lost if high clock speeds (which use lots of power) are used. More likely Ivy Bridge will be cooler but this will be because it will run on a lower clock speed (than its maximum) and have a very aggressive turbo boost. Other engineering factors than die size will determine heat. Ivy bridge will be more efficient and faster than sandy bridge. While Ivy Bridge may run cooler when not under load it will likely run at the same temperature when under load (The TDP of the chips will likely be the same). When under load, a 35 watt core 2 duo and a 35 watt sandy bridge chip will run at the same temperature (or very similar, depends on cooling systems; they will produce a similar amount of heat).
It was the transition to chips with a TDP or 45 watts from a chip with a TDP of 35 watts that is causing the heating problems in the MBP, not the transition to Sandy bridge.
