The gut reaction is what it is. When I see spelling errors in a text, AND I don't know the writer, I sometimes will jump to that conclusion.
Part of this dynamic is the idea of first impressions. In the last 4-5 months for example, I've been writing so fast and furiously to keep up with so many demands, that my spelling and grammar have taken a serious nosedive. At the same time, very few people seem to have noticed (and I'm the kind of guy people like to see mistakes issuing from

).
I've had whole weeks where I must edit even simple posts here because of mistakes I make while doing too many things at once; that is, forming a response to a complex statement (such as Bodie's views on marriage and fidelity, which I am waiting to hear more on); writing syntactically/grammatically/orthographically/semantically/logically correct statements; teaching my class (on writing Unix device drivers this week); coordinating scheduling for my company; trying to keep up on new technology, recent software patches, crap I've been backed into having to learn, etc., etc.
So I'm often typing like a fiend to take care of committing thoughts to print. When I go back to read some of it, even simple messages, I'm appalled. When I was younger, I would never,
ever let a letter go out with errors. These days, judging from email alone I look no brighter than your average incompetent writer. But people don't seem to see errors so much because they know who I am and silently forgive the transgression.
When reading someone's writing for the first time, the tendency is to examine closely; errors in usage naturally pop to the surface. You can see this phenomenon several ways. One study I did as a grad student was examining 'grade drift' in composition courses. That is, as some instructors got to know their students better, they tended to 'excuse' errors by students they felt were diligent, and find more errors in work they felt showed lack of effort.
It's in what expectations the reader places on the writer. An easier-to-see example this time of year: watch the foul calls in the NBA playoffs; they often
heavily favor the better team.
There's no such thing as impartial referring when the players and refs learn each other's style and tendencies over yers of interacting. That's why Patrick Ewing can take three steps to the hoop but your average rookie gets called for travelling.
With that in mind, it's not native spelling skill that people really care about. It's that when you've written something, there's every expectation that you've had a chance to say exactly what you mean in exactly the right way. When you haven't, the critical reader thinks you're wasting their time.
[ May 27, 2002: Message edited by: Michael Ernest ]