https://steemit.com/ethereum/@joshbreslauer/ethereum-metropolis-upgrade-in-late-septemberIn addition, the so-called Difficulty-Bomb is actually delayed and will be part of the 2nd Hardfork of Metropolis.
The Difficulty Bomb should cause an expanding difficulty in ethereum's mining.
It is an intermediate step for the gradual replacement from proof-of-work to proof-of-stake.
This is the so called Ethereum "Ice Age".
But for the next Metropolis HF in September 2017 this should be delayed to the 2nd part and replaced by a simple Block Reward reducement from 5 to 3 ether per block.
So it sounds like this first phase is going to be the block reward reduction from 5 to 3