I worried about this problem aswell, I travelled back to florida 3 times with the cuban stamp on my passport and was never questioned.
The problem I had was when we got to cuba, (it wasn't long after sept 11) They saw the american stamp in my passport and pulled me aside and questioned me, On why i'd been to the US, how long did I stay, what did I do at home, how much did I earn etc.
Im not trying to put you off, just be aware. I'd still go back to Cuba again, we had a great time.
Sam