This is friendly advice based on someone who understands this exploit. I would close the I0 Exchange. until I0 is patched and a new client is distributed. Bitparking is providing the final piece needed for I0C to be a huge laboratory. I am afraid they will find the missing piece which allow them to deploy this exploit with a lot less than 51%. Possibly against Bitcoin when NMC merges.
The I0Coin exchange (and the other bit parking exchanges) have some protection against exploits involving rolling back the block chain to steal money from the exchange. They stop processing blocks at any attempt to rollback beyond the deposit confirmation limit. This has already happened once, yesterday, at a 10 confirmation limit, and today at a 20 confirmation limit.
The I0Coin exchange is still closed for deposits/withdraws as a result of the latter while I examine the logs to see what was attempted. I'm also logging IP addresses of nodes sending rollback transactions to track down the nodes involved.
One upside of these attempts is it has given me a number of ideas on how to make an exchange safer against such attacks. I hope to implement some of these in a future exchange project and will be unlikely to be opening any further exchanges until this is done. There's just too much that can go wrong with low hash chains and exchanges.