I've noticed it and closed the pool for new registrations several hours ago. Any new miners will have to go to other pools, so this should stabilize soon. Hopefully.
Due to PoS blocks being interleaved with PoW blocks, it will be actually needed more than 51% (around those 80-90%) for the attack to be feasible with good probability - of course unless you hold lot of coins to be able to influence the PoS blocks too, helping the attack. But getting 51% of the coins would be very costly for the attacker
Also, pool having 51% is alone not a problem (except being single point of failure for miners, etc .... but there are now at least 2 other usable MPOS pools and bunch of p2pool nodes, so in case of some unlikely total failure, miners can spread elsewhere).
Pool having 51% (or with combined pow+pos coin a bit more) AND owner being evil can be then a problem (like deposit loads of coins on an exchange, then sell them for BTC, withdraw BTC and then publish longer version of the chain where the transaction from you to exchange is not present (you have your BTC and then take back your CACH), so basically it enables double spend - this is theory, but for this you need to holding back block in the pool (and this is very noticeable in the network and such bad behavior can be easily detected) top release them all at once later).
With "only" 60% with PoW+PoS coin, this attack has only very low chance of success (and very high chance of detection, regardless of being successful or not).
