Imo, diversification increases the risk rather than decreases it. (if you have money on 2 exchanges the chance of any of them being hacked/taken down increases by 100%).
I did hold all of my bitcoins from december till early june at mtgox (never lost a dime), and they even survived the recent DDOS and hacks
But i wouldn't recommend this if you plan to hold for years.
Use a non-windows OS to create your wallet, put all your bitcoins to it. Take that machine offline, and create multiple backups on USB sticks, put them in a safe and/or in a bankbox.