My housemates have just come back from holidaying in Cuba and other friends of mine have visited recently with no problems whatsoever. I'm unsure of what the situation is with regards to visas but I don't imagine it would be too tricky to find out. The one thing they did mention (I'm afraid this is all second hand info) is that there aren't any hotels in Cuba - you have to stay in private houses or something like that. Try looking for a Cuban touristboard website or asking in a local travel agents.
If you're interested, the diving is fantastic - easilly equals Cayman and Belize - but a word of warning, where at all possible take your own kit, the stuff I saw used by a couple of dive operators was a bit iffy to say the least. Locals V friendly, and Havana is a good party town.