As of version 0.3, centrally-broadcasted checkpointing is no longer a critical part of the protocol. Its purpose is to defend
the network during the initial growth period, and to help ensure a smooth upgrade path.
Central checkpointing is now being gradually weakened, and will be eventually removed,
to achieve a similar decentralization level to Bitcoin. The checkpoints exist solely
as a security measure: if something terrible were to happen, we have the checkpoints
as a backup.