I think that there is a big difference between professional, and conservative. Some places are so conservative that you MUST wear heels, you MUST wear stockings with those heels, you MUST wear a skirt suit (no pant suits) and you should not wear ANY bright colors (red, purple, pastels, etc.) In THAT sort of environment, I don't think cornrows would go over well. It has nothing to do with the professionalism of the style, and everything to do with the work environment.
I'm a professional, and I rock cornrows if I feel like it (as well as a BAA, two strand twists, etc, etc, etc) - but I also work in IT in a laid back company. I (and everyone else, I think) was addressing the OP's specific question about a very conservative work environment - not merely professional ones - largely because I feel that ANYTIME you are at work, you should be professional.