Checkpoints are really not necessary. They prevent a DoS attack where on first bootstrap somebody feeds you a bad chain and you waste a lot of time validating it before finding the good one. There is ongoing work right now on "headers first" (largely sipa's doing if you want somebody to thank) which will allow Bitcoin Core to check the proof-of-work before doing any validation, and therefore eliminate bad chains quickly.
They also make an isolation attack harder by forcing an isolation attacker to build fake blocks off the latest checkpoint, which has a high difficulty and will be costly. But this can be fixed without checkpoints by simply encoding a "minimum total work" as a weak sort of checkpoint, which doesn't favour any particular chain, just insists that enough work be done to create it.
We really want checkpoints to be dropped because they cause confusion (some other currencies use them as a stealth consensus centralization mechanism). In Bitcoin they have nothing to do with consensus, they are just a DoS mitigation, and there are certainly better solutions.
Edit: Also, please don't bump your post after only two hours, (and while I'm being a grump, please remove your garish signature).
LOL - well thanks ... I think ...
The sig didn't pay this week, but it has been regular
Sorry for the bump after 2 hours
so you reckon, screw the damned things!
:-)
