I've traveled there from Mexico (Cancun) with no problems whatsoever. I think the requirement is moreso that you cannot spend any monies earned in the US in Cuba, however if you earned the money overseas (perhaps from interest in an offshore bank account), then you could technically spend those monies there.
The process is fairly simple Once you've booked your flight on any number of carriers that fly there (national airline:
http://www.cubana.cu/html/ingles/index.asp), and arrive at the airport, you will purchase a tourist visa for USD $15-$20. When passing through customs in Mexico/Canada/etc., they will stamp your tourist visa. Ditto for when leaving Cuba. You then present your US Passport when you arrive in Mexico/Canada/etc. This will ends up with you having two entry visas for that country (Mexico/Canada/etc.) but only one exit, but I imagine that if you have a enough stamps in your passport no one will notice.