Believe it or not, Grammar Girl is suprisingly forgiving -- she says that fun, while originally a noun ("we had fun"), has made a transition in the last century or so to an "attributive noun" which can be used as an adjective ("we had a fun party"). And while old-timers may flinch at the words "funner" and "funnest", the correct way to modify one-syllable adjectives is in fact by adding "-er" and "-est" to the ends. So technically, "funnest" is grammatically correct, even if it isn't exactly accepted; Grammar Girl calls it "grating and horrifying."
She finishes today's podcast, however, with a scary note for language traditionalists -- it could very well be that Apple's usage of the word is just what "funnest" needs to go over the edge into regular acceptance. Sure, we get that language evolves, but couldn't they just have said it was "the most fun iPod ever?" Or, even more traditionally, "the most fun you'll have with an iPod?"