Via Sullivan, entrepreneur Kyle Wiens won't hire people who use poor grammar (and neither will I):

Good grammar makes good business sense — and not just when it comes to hiring writers. Writing isn't in the official job description of most people in our office. Still, we give our grammar test to everybody, including our salespeople, our operations staff, and our programmers.

Grammar signifies more than just a person's ability to remember high school English. I've found that people who make fewer mistakes on a grammar test also make fewer mistakes when they are doing something completely unrelated to writing — like stocking shelves or labeling parts.

In the same vein, programmers who pay attention to how they construct written language also tend to pay a lot more attention to how they code. You see, at its core, code is prose. Great programmers are more than just code monkeys; according to Stanford programming legend Donald Knuth they are "essayists who work with traditional aesthetic and literary forms." The point: programming should be easily understood by real human beings — not just computers.

Yes. Clear writing shows clear thought, almost always. I might not go so far as to use a grammar test for new employees, but I do pay attention to their emails and CVs.