What's the purpose of separating the 'float' and the 'cold storage'? Is it to increase security by limiting the amount of BTC exposed to an attacker who breaks in Mt. Gox?
Not sure how much exchanges normally keep in their "hot wallets", but I've heard numbers from 10%-30%. If an exchange is hacked and decides to close and go under, they can also use cold storage to send some percentage of user's balances back to them.