Just found this:
http://travel.msn.com/Guides/article...=384862&page=3
For decades, customs agents never bothered stamping U.S. passports, but today, the Cuban stamp is a kiss of death. Americans face prosecution, interrogation or hefty fines for trying to return home after an illegal visit to Cuba (fines vary, but they can amount to thousands of dollars). If you’re fearless enough to try this, understand that you’re breaking the law. And even if you avoid serious penalties, remember that the black mark will remain on your passport forever, potentially hindering future U.S. border crossings.