So let's be clear:
1. PWC, with the new checkpoints, is perfectly valid. Anyone can mine it. Anyone can use it. It will, like the other 40 or so low hash rate coins, remain vulnerable to further 51% attacks
2. An attacker was able to steal coins from Cryptsy using an exploit where they deposited coins on a forked block chain and then proceeded to withdraw those coins on a real blockchain
3. Vern has fixed the vulnerability in #2 but does not have sufficient PWC to pay everyone their full balance
While this particular attack is unlikely to occur again, a far more common one is far an attacker to do a 51% attack, deposit coins to an exchange, immediately convert to BTC, and then withdraw the BTC. That attack is easier to pull off and much harder for an exchange to protect against.
Anyone trading alt coins on an exchange needs to be aware coins can effectively be stolen from an exchange in this manner resulting in a loss of capital for the traders. That is why there are very few exchanges that allow low hash rate coins to be traded.
You don't get how hilarious the story is. I've spent some time on analysis of the Powercoin's block chain. This is not a single exploit or security breach, this is something really outstanding
They say someone with huge hash power took control over the network, so here we have a 51% attack. That evil guy double spent his coins through Cryptsy. He forked the block chain, deposited coins at Cryptsy, mined new blocks until it confirmed, then switched to the original block chain, withdrew coins to his address, and continued to mine until the withdrawal confirmed. It seemed no one cared to process such high deposit and withdrawal manually, so the evil guy got away with over 2 million PWC. Cool, yeah? Now they feel pissed off extremely and ready to destroy the whole currency to make sure the hacker never profits. Good guys with PWC deposits at Cryptsy are going to receive some kind of compensation, other good guys out there get nothing. And the story ends.
Wait, 2 million coins moved at once? Get ready for real fun now. They weren't hacked once or twice. They were hacked a couple dozen times! The bad guy worked hard 2 days long forking and orphaning the block chain, depositing and withdrawing coins. The 1st attempt was at block
#29294, time stamp 2013-06-15 00:38:00. He started with some 20K PWC to double spend. It worked out, cool. He tried it again a few more times. No problem. The dude decided to raise stakes. Again and again. The last double spends were over 200K each:
#29669 (2013-06-16 11:55:17) and
#29706 (2013-06-16 15:42:35).
His highest withdrawals went to this address:
p5p53rioyjqyuwbySUzZAeX2JU4778yECZ2 days under attack and no one gave a shit! They didn't care even to increase the number of confirmations for deposits, say, to 100 like BTC-e did with Feathercoin recently. Now they tell stories and the community is at loss.