Dictionaries merely reflect usage, Nigel, and they don't pass judgement. Clearly, "hippopotami" is in currency so dictionaries would report it. What dictionaries won't tell you is that you might look a bit of a twit if you use "hippopotami" in anything other than a humorous context, for example, if you were writing a treatise on the biology of, er, hippopotami... I do that fun thing meself, frequently, whenever more than one hippopotamus is on the radar.