Gwyneth is right, here. Criminal law doesn't depend on a common law body of cases like civil law does. It's statutory. So if there's a statute on the books making something illegal, it is illegal whether or not there's been a prosecution for the specific flavor of the crime in question, at least in the U.S. Here, the relevant statute is 15 U.S.C. § 77b (a)(1), which defines "security" pretty broadly. There are lots of debatable questions in virtual law, but this one -- whether securities laws apply to in-world securities -- is fairly straightforward. If you are buying a share in a company for anything "of value" it is a security, and there are all sorts of regulatory devices that securities dealers have to comply with, none of which are being followed by the virtual exchanges. One could argue that Lindens don't have "value" but it hardly seems like a winner, given that there's a web page where you can readily convert them to real world cash.