There's no checkpoints in Bitshares, users will be able to opt out checkpoints in Peercoin 0.5, and I don't know enough about NxT checkpointing to comment.
Interesting discussion you have here. I just want to inject some missing info which is too long to search.
Almost quoting Come-from-Beyond here. NXT in its bootstrapping phase (which is now) has rolling checkpoints 720 blocks deep (so called automatic re-organization of the blockchain). After the bootstrapping phase is over (could be by the end of 2014, at least that's the plan), Transparent Forging is supposed to be implemented which sets rolling checkpoints to 10 blocks deep. And CfB acknowledges that Transparent Forging is somewhat similar to what DPoS does, but at the same time it's different. In DPoS delegates are voted by users. In Transparent Forging mechanism of NXT the software will transparently define the next forgers of blocks with the algorithm, which enables transactions being sent directly to those forgers, which makes possible a very fast block generation (15 seconds - 1 minute).
The number of successful forgers in NXT since Genesis block varies over time, but on average is 250-300 as analysis of public keys in the blockchain reveals.
