Well, I wouldn't say they're completely orthogonal, knowing how a word is spelled can sometimes give insight into the meaning of the word. I think they're mostly orthogonal though; it's fairly common for people to know what a word means without knowing how to spell it, and on the flip side there are people, like Scrabble players, who know how to spell a lot of words which they don't really know the meaning of. I've heard of one guy who is a champion French Scrabble player who can't actually understand French.