Am I crazy? Well, yes, but that is beside the point.
By fractional reserve, I don't mean that they should start making loans or other investments. Rather, they should only keep only a fraction of their bitcoin reserves available for immediate withdrawal - enough to provide day-to-day liquidity.
The bulk of coins should be stored in multiple encrypted offline wallets (with multiple backups and a nice big gap of air between these wallets and the internet). These offline coins would not be available for immediate withdrawal, but they would be safe!
Coins could be sent to the offline wallets any time, and coins could be returned to the pool of available coins manually as needed. The concept is analogous to a bank vault. Most of the cash at your bank is not sitting in the till in front of the teller, but is securely stored in a vault which can only be accessed at certain times under heavy security.
Users of any such site should also have the option of storing a fraction of their coins in such an offline wallet. You could even give each user their own "offline address" where they could securely store coins for the long term, and they could see the coins in block explorer. Bringing the coins back online would require extra verification steps and built-in delays. This is analogous to a safety deposit box, and you could even call it that.
I'm sending PMs to the exchange reps, asking them to comment on this thread.
DCM,
Very good point, but for newer exchanges our wallets see a lot of activity day-to-day. For MT.Gox 2% of the wallet is sufficient to cover daily activity, for us it is more like >30%.
As we move forward, this will of course be a top priority.
In the meantime, to stay ahead of natural / man-made disasters and hackers we have following measures in place:
1) Secure data center designed to survive Cat 5 hurricanes
2) Connectivity to three telco backbones
3) Redundant power with 2 Caterpillar diesel generators to support the data center power
4) Physical security for servers
5) Well defined chain of command and separate ownership for Database and Wallet
6) Multiple, offsite backups
7) Nightly security audits
Daily D-Dos simulations
9) Scheduled White-hat penetration tests
Hope this helps,
Keyur