Maybe. IANAL, but if you are in the U.S. and advertise to other people in the U.S. then its SEC jurisdiction. Even if the website is across the globe.
If you are not in the U.S. and advertise to people in the U.S., then it will come down to the local country willingness to cooperate with the SEC. I think the FSA is more than willing to play ball in an investigation for instance. (U.K.).
If you are not in the U.S. and you advertise only to people not in the U.S., the the SEC has absolutely no jurisdiction to do anything to you. Some other authority might though.
At least this is my understanding.
A lot of this boils down not so much to law, but to political and practical questions - examples might be Julian Assange, Kim Dotcom, or Sonny Vleisides (the guy at BFL whose arrest/conviction created a lot of excitement here). In each of those cases, the US decided that what those people did, while they were outside the US' territorial boundaries, was offensive enough to the US that they were willing to devote considerable effort and resources to catching them and punishing them. I do not mention this to argue about the legitimacy or the validity of the US' actions, but to simply point out that they occurred. Someone else who achieves a high enough profile, though offshore, may find themselves in similar circumstances.
The US has always had a relatively expansive view of the reach of its regulatory jurisdiction, and has always had a very limited view of the reach of the US Constitution's protections of individual rights. (E.g., if you are in another country, the US can punish you for what you do there, but you cannot expect the protections of the US Constitution regarding search & seizure, interrogation, counsel, etc.) Both of these things have become even more exaggerated since 9/11/01 - to be honest, I haven't bothered tracking the precise contours of exactly what the US thinks it can do these days, since successive presidential administrations have been eager to exceed those contours, and the courts have been willing to go along with the project.