This prompted Jeremy to write a rhyme, Plurals of the many-footed, in which he posed the question: “Would they think us all wusses / not to embrace the octopuses?”
I responded with eight hurried lines of nonsense, reproduced here for the pleasure (or more likely pain) of posterity:
Octopodes, they swim not run,
They have a beak but not a bill.
Larger things they tend to shun,
Littler things they tend ta kill.
But what an octopus might think –
Whether singular or plural –
Is hidden in a cloud of ink
Obscuring all things cephaloneural.
To these I will add this useful discussion at bradshaw of the future, who notes that “usage trumps etymology every time”.
I prefer the plural octopuses, except where rhyme or rhythm warrant one of the alternatives. An adequate summary of their merits would require several paragraphs, which would be pointless given the links above: all are worth a moment of your time, if the topic interests you.
Wikipedia also has a decent, well-referenced account.