As far as my understanding goes, you can visit Cuba, but you cannot spend money there. The USA has an embargo on Cuba. Wikipedia has a pretty solid article on it. You need a license to actually participate in commerce... but since that includes buying food you in essence cannot go to Cuba.
Licenses are released however. I have a Cuban uncle who goes back once a year. The valid reasons as per wikitravel are:
Professional journalists on assignment in Cuba
Full-time professionals conducting academic research or attending
Persons on official government business
The following are issued on a case by case basis:
Persons visiting immediate family in Cuba
Full-time graduate students conducting academic research to be counted
toward a graduate degree
Undergraduate or graduate students participating in a study abroad
program of at least 10 weeks in length
Professors/teachers employed at a US institution travelling to Cuba to
Persons engaging in religious activities
Persons engaging in humanitarian projects
Persons engaging in non-profit cultural exhibitions
So in summary, is it open travel? No. Is it possible? Yes. The most common way around this that I have heard is "travel writers."