While I'm all for a history lesson (and the double-I octopii is indeed simply incorrect) I take issue with anyone insisting that "octopi" is wrong:
1. Language is neither static nor a series of rules to be blindly followed. The way a word was pluralized 1400 years ago has limited relevance today.
2. As noted just about everywhere, "octopodes" looks insane in any modern English sentence because we don't pluralize any other word that way. It also moves the emphasis to the second syllable. Thus it manages to make everybody's life harder for no benefit, a favorite pastime of the sort of people who would suggest this pluralization.
3. "Octopuses" feels stilted, and while it is correct, I thoroughly empathize with anyone uninterested in using a four-syllable word with three consecutive unstressed syllables in a sentence. Therefore it makes sense to create a shorter pluralization, and we can do this by analogy to other English words!
3a. We are not speaking Latin. If "-us" to "-i" is a valid pluralization of other English words, then it makes sense for it to be a valid pluralization of this word. While this pattern can be used irresponsibly ("bus" -> "bi"), using it for the three-syllable "octopus" is non-destructive. It preserves the structure (and the meter!) and thus makes a lot of sense.
4. To come back to "double-I octopii is simply incorrect": It's wrong because it's trying to be pedantic but uses the rules wrong (as noted in the wikipedia reference above). If, in 700 years, I were still alive, English were still spoken, and some band of idiots had managed to make "octopii" the most common pluralization, then I would begrudgingly accept it per point 1 above, but until then, no.