I know some smart people who aren't good spellers but no good spellers who aren't smart people, though I know there must be idiot savants who can spell every word in Webster's Second – decent folks won't truck with Webster's Third – but can't use those words with any skill. I'm always a little taken aback by good writers who can't spell, as it seems somehow those two skills should go together. Words are, after all, the tools of a writer, and not knowing how to spell them is to use defective tools. I've quit remarking upon bad spelling in the Vicad, but it's still frequent. Picking on the Vicad is like stomping puppies, too easy.
Virginia Heffernan, blogging in the NYT, has some stuff to say about spelling:
Bad spellers are a breed apart from good ones. A writer with a mind that doesn’t register how words are spelled tends to see through the words he encounters — straight to the things, characters, ideas, images and emotions they conjure. A good speller, by contrast — the kind who never fails to clock the idiosyncratic orthography of “algorithm” or “Albert Pujols” — tends to see language as a system. Good spellers are often drawn to poetry and wordplay, while bad spellers, for whom language is a conduit and not an end in itself, can excel at representation and reportage.
Read all of her insightful comments here.