When I was living outside the US about 10 yrs ago I almost had the opportunity to do some work in Cuba. Since I had a US passport, I had to do all sorts of checking the the embassy about this.

What I learned is that US travel and commerce is not necessarily prohibited, but the exchange of US dollars for goods and services. Since I was paid in foriegn currency at the time, this was not really an issue.

Further, if I did bring any items from Cuba into the country for personal use in the future (cigars), and could prove that they were paid out of non-US funds, there would be nothing that Customs could do about it.

Of course I had this all documented at the time, but then the work never came through. Hope to get there someday when we have a more lenient policy toward that country...maybe like we have with China.