they have some kind of approval for scientific, educational or humanitarian reasons. Not much point to the embargo that I can see, but hey... maybe its a holdover from the missile crisis back in the 60s. Let's see... you guys play nice with Japan now, so eventually I'm sure you'll kiss and make up with Cuba too.
Also, don't be fooled by travel agencies (some in Canada, too, which is embarassing) that tell you they've got a "loophole" whereby they take your money and mail you an all-expenses paid "invitation" to visit Cuba. According to good old Uncle Sam, that's a no-no as well.
My buddies simply go and don't bring anything back... never had any problems as far as I've heard, but I'm sure there's a first time for everything.
One last thing - as I understand it (and I may be wrong about this), its NOT illegal for Americans to go to Cuba... it would be unconstitutional (in the land of the free) to restrict a citizen's right to travel. Unfortunately, it IS illegal for a US Citizen to spend money while they're in Cuba! (I thought that was a great loophole myself, actually - way to go Uncle Sam!).
Hopefully there are players from other countries represented on the Geocaching board, so I'm hoping for the best.