I learned about the existence of the word "octopodes" yesterday and had to go on a research spree.  Here is what I've gathered: octopuses, octopi, and octopodes are all acceptable pluralizations of octopus.

• Octopuses is the English pluralization.
• Octopi stems from a popular misconception that octopus was originally a Latin word - it's actually Greek - and used to be considered grammatically incorrect but has since been used enough to become a viable option.
• Octopodes comes from the original ancient Greek pluralization.

I have also just learned that despite the accuracy of the above statements, my spell check firmly denies the existence of both octopi and octopodes (and, in fact, pluralizations).  Nevertheless, I'm going to be testing out octopodes as my preferred pluralization for a while.  It has a nice sound.