It's not always so much intelligence, but my opinion on how much they value how they present themselves. I was reading a wank yesterday where a guy was flaming anyone who pointed out his spelling errors were so flagrant and rampant that you couldn't understand his point. He kept whining "but I have a thing, and I'm a really bad speller!" Fine, whatever. You have a condition, you spell poorly, fine. There's no excuse for not spell-checking in this day and age, and if you know you're not good at spelling and you are going to make no attempts to correct it, I'm going to treat your statement with the exact amount of concern you demonstrate.

An occasional typo? Sure. Everyone makes them. But a consistent disregard for your reader means, to me, that your words are not something I need to give a shit about. ::shrug::