Checkpoints were made optional via an easily accessible checkbox in v0.6. Developers still believe checkpoints have value as a voluntary option, especially if we do something like multisig checkpoints in the future. Turning this feature off is currently as easy as unchecking a box in settings>options.
Yep, I knew that. However, even if some nodes disable the centrally broadcasted checkpoints, some will still follow them - and that converts them, in theory, in a vulnerability (it's not that the checkpoint key owner had to be an attacker, but it could be the case that his checkpoint server gets "lured" into an attack chain by an attacker), above all, if the checkpoint option is used by nodes that are minting.
I would prefer a rolling checkpoint scheme like in NXT and many other PoS cryptocoins. Or even Vitalik's
exponential subjective scoring.