a slow transition is planned for eth. The first implementation will come with the casper release this year, but proof of work will stay there and slowly phase out to keep the network secure.
I doubt that it will even happen this year, if ever.
Even if it does, it will create a PoW ETH hard fork as many don't believe PoS is sufficient. It's open source software, so the Difficulty Bomb can just be removed or delayed forever. It's not much of a Difficulty "Bomb" since they already delayed it once (twice?).