What exactly is the plural form of octopus? I've heard octopusES and octoPI from many different places. At m-w.com and dictionary.com, they have both down as pl. forms. They both can't be right, can they?
Linguistic sticklers will tell you that it should be octopoda, in keeping with the Greek origin of the word. The plural i or ii is from Latin. Scientific names are often based on Latin words, so this may be why scientists use it in the stuff Greg reads. You can confidently say octopuses, too. Or you can skip it entirely and refer to "all members of the genus Octopus."
Melissa's right, infact...OCTOPI is the false plural for Octopus.
Although it is often supposed that octopi is the 'correct' plural of octopus, and it has been in use for longer than the usual Anglicized plural octopuses, it in fact originates as an error. Octopus is not a simple Latin word of the second declension, but a Latinized form of the Greek word oktopous, and its 'correct' plural would logically be octopodes.