Services have existed with margin/option trading, but some of them have shut down because they were hacked. New ones that come out every day are too insecure to be trusted.

In fact, just today a margin trading website was exposed as having tons of SQL injections and storing user passwords with unsalted MD5 hashes. Until somebody can step up to the plate stability will be an issue.

Which reminds me, I have a rather nice domain name and experience with bitcoins and security, if only I had the financial/regulatory know-how...