Can Planck's temperature ever be reached? Planck's temperature is theorized to be the highest possible temperature in the universe. But, has it been reached? Or, will any object or phenomenon ever reach this absolute temperature?
 A: The Planck temperature is formally reached in the last stage of the evaporation of the black hole. The minimum black hole – that already decays to the final "several" particles of the Hawking radiation – formally has the Planck temperature. However, because such small black holes are also rapidly disintegrating, the system is far from equilibrium, so the temperature can't be accurately well-defined in any way.
There can't be any way to achieve an equilibrium state at this high temperature. The reason is that particles/quanta at such high temperatures carry huge, near-Planckian energies (except for hypothetical elementary particles with rest masses greater than the Planck mass, but those really don't exist but these heavy elementary objects are inevitably black hole microstates) and there is a large probability that they collide. 
Such (trans)Planckian collisions lead to the production of black holes and the black holes rapidly "cool" the system because the temperature of a black hole is much smaller – such that the thermal wave length is comparable to the black hole radius. So Nature automatically cools itself near the Planck temperature. In this sense, it is really a "limiting" temperature although this claim isn't as sharply defined as an analogous statement for the speed of light, as far as I can say.
