The Plural of Octopus

Considerable discussion among the pedants in our group about the proper plural of "octopus." I had never really thought about it, and never looked it up. Apparently,"octopi" is incorrect. Indeed, my spell-checker has just flagged it as an error.

The correct word is apparently "octopodes," although "octopuses" is also acceptable (and the only version my spell checker -- and Blogger's -- seems to accept).