I am not aware of any. Whilst I like to think of such not-yet-existing services as simply waiting to be provided soon, there is a significant barrier to creating such a service. A broker wanting to offer taking bitcoin deposits as collateral for your stock holdings would need a reliable and classical-finance-compatible way of converting this collateral into USD. This would be needed in case stock price or bitcoin price movements come close enough to using up all the available collateral. With stock exchanges, the broker can do such a service whilst not taking any risk not somehow addressed or knowingly tolerated in the world of finance. Existing bitcoin exchanges do not fit into this because they operate slightly differently from regular stock or foreign currency exchanges. Hence there is additional risk that, whilst apparently acceptable to customers of existing bitcoin exchanges, would probably be seen as utterly unacceptable on many levels by a broker (regulatory, reputational, own risk assessment, ...). Basically, I think such a broker would have to find a bitcoin exchange operating the same way and regulated in the same way as a stock exchange, to be able to make it square with the rest of his business. There may possibly be exceptions among brokers only catering to accredited investors---those are people rich enough that the law automatically trusts them to enter financial business otherwise prohibited for customer protection reasons. But if you qualified as such, I doubt you'd go to a website for advice about finding a broker...

I've looked into 1broker and while there is quite a bit of good feedback about the site and the GUI is very clean the 4 guys who run it are students. I ran a hedge fund for many years and know a fair bit about what they do which is CFD trading. The short answer is, and it sucks in a way because eventually they will tank. A black swan event will come along and take them out. They don't understand hedging (which the main guy says he uses "when he feels it is necessary") That is not hedging. Hedging is a considered and consistent strategy you put in place before you start running your trading business. Not something you do when the mood strikes you. However, it is my belief that eventually a professional group will build a professional platform that supports digital currency as security. The world has to catch up with the idea and i am certain it is coming. I've got a few BTC I wouldn't mind putting to work. Anyone else?

pyramids provided a really good answer. I would like to add though that as time passes and Bitcoin prevails as an alternative viable virtual currency due to ever growing acceptance/usage then Investment/Financial Firms and Banks may well consider accepting Bitcoin. They would have to remain in business. But you can bet there will likely be BIG FEES for accepting it and stringent margin requirements with minimum 50% in all likelihood and doubtful will allow lending on margin over and above this.

You may want to give First Global Credit a try (www.firstglobalcredit.com). They provide stock trading facilities and provide a DMA trading platform (all trades are executed directly on the relevant exchange) with all positions secured using Bitcoin as collateral. I've tried their simulated trade platform and it seems to do what it says on the tin.