I'm guessing part of it might also be in order for Mt. Gox to keep its float steady. They have a certain amount of BTC available at any one point, with the rest in cold storage. If someone were to withdraw a huge amount at one point, it'd drain the float and everyone else would not be able to do anything.
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?
Correct. A cold storage scheme might be private keys printed on paper and stored in a safety deposit box, for example. Then, if their network is compromised, they won't lose everything.
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.
