My take on this is that the regulators are going to give brokers a hard time about bitcoin because of money laundering issues. The stress that financial firms relieve from regulators about AML is hard to appreciate if you haven't worked on the inside. I admit my ignorance on AML & bitcoin, but is there a way to monitor this?
I think the main AML issues are with
source of funds. If you allow only trading and withdrawing via bitcoins then it would trigger less issues than if you also allow accounts to be funded via bitcoins, exchanged into fiat currencies and then withdrawn into bank accounts.