Theories, of course, abound. It is generally agreed that "sploodal" first appeared in northern Europe sometime in the late 1990s, and one theory is that it derived from the Norwegian word "sblødl" (a rather mild, yet tasty, firm white cheese). Why a Norwegian word for cheese should come to be associated with all things cool, hip and desirable, however, has never been adequately explained, except perhaps for the suggestion that it was originally used ironically and that non-Norwegian listeners simply didn't get the joke.
Another theory is that the word is actually a shortened form of the phrase "split poodle", which is slang for a particular sexual position, the details of which I won't go into here. One major flaw in this theory is that in almost all cases the word has, in fact, been spelled "Sploodal" and not "sploodle". It is possible, of course, that the spelling of the final syllable was purposely altered in order to make the word's origins a bit more obscure to one's parents, but I think that's stretching things a bit.
One final theory that has been making the rounds lately is that the word derives from "splood," which is an obscure term describing a certain type of "ornithological dejecta" (that's bird poop to the rest of us). This is almost certainly a coincidence, however, since the term really is truly obscure, even among ornithologists.