TRC has shown that there is an additional computational vector for attack: A difficulty based attack. You can make the chain diff level so high that it slows the rate of production of blocks to the point where the network is at a standstill. Could happen for Bitcoin although you'd need a crazy powerful system to do it.
What would happen with ppc ?
1. i point my popular btc pool to ppc for a while,
2. difficulty starts skyrocketting
3. miners leave
4. repeat 1+2 when needed
how long would it takes to POS to bring difficulty down to a reasonable level back ? Isn't POS efficiency related to the number of running nodes and their coin age ?