As far as I know, the most accurate plural would be octopodes, which I've never heard anyone use in conversation -- understandably so, as it sounds insufferably pompous. I always use octopuses; I figure that since octopus has become a distinctly English word, it's best to use an English plural form.
I mean, think about it: If you're living in an English-speaking country and you go to Starbuck's (or whatever your fav coffee place may be) with three friends, you're going to order two cappuccinos and two espressos, not two cappuccini and two espressi.... even though the latter are more correct in the original Italian.
Octopi, of course, is another story -- and a source of infinite frustration to those of us who have been corrected by self-styled grammar nazis who are under the mistaken impression that octopuses is wrong. But here's where we get into a linguistic grey area: As it happens, when an incorrect term is almost universally used, it eventually becomes acceptable. That's how languages evolve.
For example, look up the original meaning of such common English words as "nice" and "silly" -- see how they evolved into something totally different from their original definitions? Yet we all use them today to mean "pleasant" and "foolish", respectively. More recently, look at how we use words such as "awesome" and "cool" -- sticklers for precision may rant and rave, but ultimately common usage prevails and such alternate or colloquial meanings become completely acceptable.
So from what I can see, octopi is becoming less and less wrong as time goes by. I still think octopuses is awesome -- but hey, if someone else wants to use octopi, that's cool.
~ Tani Banani ~