Vitalik Buterin writes:
Assuming ~20% participation in PoS and current prices, you could stall or break the network at a cost of ~$7 million. However, unlike PoW, recovery is simpler: Ethereum just hard-forks to delete the ether of the malfeasors (if /u/vladzamfir does his job correctly, all currently online nodes may even be able to coordinate on this automatically) and moves on.
If such a hardfork would happen, would a Slock.it lock easily be able to go along to the new fork or could it stay stuck in the old chain?