When Fidel Castro lets go of the reins of power — however it happens — visiting Cuba will become attractive to U.S. citizens once again. However, the laws that have prevented easy travel to the Caribbean nation since 1963 won't go away overnight. Castro's probable successor will be his brother, RaÃºl, and will most likely follow current policies, which the US government disagrees. So, he too will need to step aside too. However, with good luck and other democratic changes like free multiparty elections and release of political prisoners, US policies would likely change — as would interest of potential travelers to Cuba.
Of course until that occurs, you can do as thousands have done and go illegally via Mexico or Canada, but that certainly isn't recommended. To go there legally you need to apply for a special permit from the Office of Foreign Assets Control. Contact USA Cuba Travel (877-462-8221, usacubatravel.com) for application guidance or more information.