This is a common symptom of aspergers syndrome. That could also explain why your daughter appears to have a learning disability.
if it makes you feel any better, my nephew did this for YEARS. maybe 2 years. and he grew out of it and he is fine. i would get it checked out but im not sure i'd get too torqued out about it yet.
My sister is 11 and she has been toe walkin for all her life. Now she is unable to walk normally without it hurting. They gave my mom an option of having her were metal braces at night or surgery to cut the tendons. It was not a symptom of any other underlying medical diagnosis but they will take x-rays and such of the legs to make sure there isn't any thing else wrong.
I knew a little girl who used to walk on her toes like that, she was a child of friends of the family she outgrew it around 6yrs old or so, her parents were told it was not anything to worry about. she was normal enough, not slow or anything. :) They do outgrow it.
Take them to your ped and bring it to his/her attention. It could be a tendon problem that's preventing them to stand flat on their feet. Do they do it consistently or occasionally?