PPCoin and other coins use Proof of Stake in addition to Proof of Work. Stake is how long a coin has been idle multiplied by its value. If you've been holding many coins for a long time, you can spend their coin age in lieu of poof of work. The fear is that someone will hoard coins, gather a bunch of age, and then double-spend and use his accrued coin age to give each spend 6 confirmations immediately, without needing CPU work.

It's a valid concern, in my opinion.