Diablo 3 is receiving its 1.05 patch tomorrow during a 10 hour window of downtime. Once that's up, you'll be able to try your hand at the new Monster Power difficulty.
The new "mode" of sorts is accessed by crafting something called the Infernal Machine which then spawns mega versions of bosses in pairs of two for you to slay, including the Skeleton King and Magda, Ghom and Rakanoth, and the Siege Breaker and Zoltan Kulle.
Each of the bad guys sports new moves, too, and all are said to be at about Inferno level difficulty, so clearly, this is not for the faint of heart. The rewards are great, however: gobs of gold and loot, plus the great Hellfire Ring, which grants a 170-200 boost to a stat of your choice and a 35% experience boost, and includes other randomized properties.
Hit the source for full details.