@MikaMiaka: Nana is voiced by PAKU Romi, who was born in 1972 in Japan to Korean parents, and who won the first annual Seiyuu Award for best female seiyuu of 2006 for her role as Nana.
She is also famous for such roles as Loran Cehack in Turn A Gundam, Edward Elric in Fullmetal Alchemist, Teresa in Claymore, and Haraken in Dennou Coil.
I agree with you that she is really a great voice actress. If I had to pick the best female seiyuu of all, it might be her. Her voice is precise, but also can convey deep emotion. Here's a pic:
@yezhangquan -- Yes. The later volumes of the manga are amazing. I really hope we get a new anime. As I hope we get a new series of Skip Beat, which is a different kind of story, but another good shoujo manga that the anime covers only a small part of.