It's not an issue when PoS becomes common as there should be more PoS blocks than PoW blocks, so you wouldn't see long consecutive PoW blocks to be orphaned by a malicious PoS minter.
Checkpoint should be enabled for any ppcoin forks.
Actually, it's an issue, because it takes some time to make PoS common for the chain. This makes network vulnerable until there is not enough stake weight in opposition to attacker. And checkpointing is not more than workaround for this.Checkpoint should be enabled for any ppcoin forks.
Also there is another problem... Unlike PoW hashing rate, stake weight could be used multiple times without any overhead. It allows attacker to repeat his attempts to generate consecutive stakes until he will get lucky enough. And there is high probability to success without holding large stake, 20-25% of total weight (not coins) will be more than enough.
You can retry your attempt to generate consecutive stakes 1000, 50000 or 1 million times without any problem, using the same outputs. You should always hold it in the mind.
So, with the current weighting scheme you should always have checkpointing enabled, even if there is enough total stake weight. You can't remove checkpointing without altering block weighting implementation, because it's dangerous. And you can't alter it without making system less energy efficient...
But I think that energy-efficiency could be partially sacrificed in favor of security.
// EDIT
+1
