And you get a whole lesson in Latin grammar:

http://www.straightdope.com/mailbag/mplurals.html

I guess that's pretty much the long and the short of it, eh?

There must have been a lot of sailors with two penises,
rember the trousers with two flys?

Short questions don't always have short answers, but try explaining that to some script kiddie who insists on virii instead of viruses. I note Cecil did not mention octopodes as a learned plural.

He did mention it. Just didn't say it was 'learned':

"Octopus is another of these third declension nouns in Latin, borrowed from Greek. The Latin plural is octopodes, which is acceptable in English, but octopuses (or even octopus) seems more at home in English."