As a Brazilian, I agree with ricardobeat. I don't think I ever saw a situation where saying 'rabo' as a slang for ass would be considered even slightly prudish or childish, but always at the very least explicit. The african variations for 'ass' are much more tame (used on TV broadcasts, for example).
Oh, I didn't know that. Well, that's a good reminder that pt and pt-BR have several vocabulary differences. A few more I can remember would be the words "bicha" (pt: queue, pt-BR: male homossexual) and "durex" (pt: condom, pt-BR: a kind of adhesive tape).
