Actually, at the risk of being a biology pedant, if I'm not mistaken, both "octopi" and "octopuses" are incorrect, and "Octopodes" is correct. It's all because octopus is based on Greek, not Latin, so you don't have to turn the "us" into an "i" when pluralizing. Octopuses isn't technically correct but wormed it's way into common usage/dictionaries just like (*shudder*) "irregardless" and other not really correct words.
But truthfully, for common usage, I doubt most people care on whit at all which one is correct, and I'm sure not going to lose any sleep over it


Cheers,
Colin