J.-
I hate to say it, but the chances are that she already knows everything.
I remember reading the book "Where did I come from" when I was 8, but kids had been talking about sex in 2nd grade for a long time. It was just confirmation that what someone said was true.
My husband had a different experience. He grew-up in a more sheltered family that was very religious, and the subject of sex was forbidden. He has no recollection of a conversation with his parents, received no discussions in health class at a private Christian school and only recalls a pamphlet someone gave him.
I'd have the conversation, and I'd keep it open for discussion any time she has questions she wants to ask.
I know girls who lost their virginity in 7th grade (at 11 years old). I didn't have my first kiss until I was in 10th grade because I was so nervous.
Believe me, they're talking about it in school, and I'm of the opinion it's better to learn morality from you, her mom, than the other kids. I'd explain what's great about it and why it's important to wait until she's an adult because of the unwanted consequences.
If you approach it openly, biologically, and honestly, I think you'll be leaps and bounds ahead of many other families.
Oprah did a show about it last year with an 11 year-old guest correspondent. I'd recommend either going to her website or seeing if you can find the show on Hulu.com to see how they approached it. The parents were in complete AWE how much their kids knew and how much they wanted honesty and to have the converstion.