I just overheard the my dd's first grade teacher talk to another parent. She was reassuring this parent that she usually has 3/4 kids who are not reading well at the beginning of 1st. This is in a gifted school (top 3% FSIQ). There are kids in the classroom reading BOB books.
I don't know when you "should" worry about not reading, but I would think that before 6 and in K, it is still very normal. I did think (for other reasons) that my DD might have dyslexia when she was 4.
I found this very informative, it talks about dyslexia in gifted kiddos.
http://mislabeledchild.com/html/Library/DyslexiaReading/Stealth_dyslexia.htm
