You can still get 'caught', even if your passport isn't stamped. Too many entry stamps into Mexico, or where ever you travel to Cuba from.
I love Havana, was there for the International Jazz Fest a few years ago, before the current administration tightened the loopholes on 'cultural exchanges'. Hey, I think Jazz is cultural.
It's an awesome city. Every morning I'd wake up, go out on the balcony and just stare at what really looks like a smaller US capital (or is it capitol?). Obviously built when things were better between the US and Cuba. It's amazing to be in a city of over 3,000,000 people and to see such few cars. No traffic hassles at all. No freeway system. It's going to take some real work to make Havana a 'modern' city. It would be nice if it could retain it's magic.