We gave my daughter her first cell phone at the age of 10, with very strict conditions - we can access it anytime. We explained to her the reasons for giving her one, and that if she was constantly on it, it would be confiscated. Just over a year later, you would hardly think that she had one. The reason for this - she is modelling our cell phone behaviour. It is a tool to be used to enhance life, and not something that replaces face-to-face" communication. We also stressed that we were giving it to her as a way for us to get contact her, not because "all my friends have one". She was, in fact, the last child in her class to get one by at least a year.
On the occaision that I need to get hold of her during the day, I have to remind her to take it to school, as well as write a note to the teacher.
If you child is mature enough, and if as parents you are able to model the behaviour that you expect from her around a phone, go for it. It will take a week or so for the novelty to wear off, and normality will start to assert itself. Only you know your child, so follow your instinct.