About a year ago, I opened accounts with BTCT.co, a company that made exchanging securities for Bitcoins a possibility. However, this was impractical - Bitcoin was extremely volatile, and the price of the security often reflected the price of Bitcoin itself.
However, now with Bitcoins relative stability, investment would be viable. Unfortunately, all the old exchanges I knew about are now shut down or inactive. BTCT, Havelock Investments, MPEx, and BitFund.
Are there any new security exchanging platforms? Has this died down?