In short: PoS migration of Ethereum is in progress and will happen in stages. "Stage 0", which doesn't have any practical effects for smart contracts, is expected Soon (tm) (a few months probably).
It'll most likely be a couple of years until the transition is done so that smart contracts like Urbit can/will run under PoS.
It'll most likely be a couple of years until the transition is done so that smart contracts like Urbit can/will run under PoS.