In general yes. But with a special visa it is permissible, and although general tourism does not qualify, for reasons such as athletic events and special diplomatic issues, a visa can be obtained.
My team and I all have this special permission to travel depite the embargo. In fact my team has been sending guys down there to race in the early season for years. Anybody read Joe Papp's diary of the Vuelta a Cuba on Cyclingnews.com? He's on my team, and as you can see, some of the guys have already been down to race there this year (I wasn't allowed to do the Vuelta a Cuba becuase I'm too young).
- Maurizio