I haven't heard of any security issues so far with OpenSim virtual currencies.
Currently, there are three main options.
1 -- A commercial grid owns all its own servers and runs the currency itself. Grid users never have access to the server code, so the currency is reasonably secure. (This won't work for open grids where people can connect regions they run at home or on their own servers.)
2 -- A grid uses PayPal, where the final payment confirmation step takes place on a website. This option is as secure as any other kind of Web-based payment. Meaning, pretty secure.
3 -- A grid uses OMC, where again the final payment confirmation step takes place on a website. Here, the security is as good as the OMC website -- since the guys who run OMC also run Virwox, one of the biggest virtual currency exchanges out there, doing millions of dollars of business, it's probably reasonably secure as well.
I would still recommend that people don't keep large balances in a virtual currency -- any virtual currency, including Linden Dollars, for the following reasons:
* You can, at any time, lose access to your account and your money, for a wide variety of reasons.
* A grid can, at any time, change the value of its currency, change the way cash-outs work, or eliminate cash-outs altogether.
* A grid can go out of business. This happens to small grids a lot, but has been known to happen to large companies, as well. If that happens you may, or may not, be able to get your money back.