Bitfinex was required by US regulators to switch from cold storage to segregated per-user wallets. Something to do with margin requirements for commodities exchanges.
Can you link to more detail about this? This doesn't seem like a requirement that would be enforced for other "commodities" exchanges? Also, it doesn't seem impossible to set up "per-user" cold storage?
Correct. Here is the source: "CFTC Regulation Prevented Bitfinex From Using Cold Storage" http://imgur.com/O46UNix
I think you are probably right, the CFTC is not going to mandate a specific technical approach. Still, an exchange could very well make a poor choice in an attempt to comply with regs whose intent is focused on something very different from the risks of wallet theft.
This reminds me of when Mt. Gox was hacked and it was later found out that US Homeland Security officers (who were undercover) stole coins and laundered them through another exchange. Seems like someone in the CFTC wanted some coins for themselves..