Dear Jeff,
This is a common phenomenon, and will likely corect itself. It certainly is not a reason for alarm. Mention it to your pediatrician, who has the best handle (next to you and your wife) about the specifics of your daughter's development. And remember, she is at the early end of the continuum in learning to walk. If your pediatrician has any concerns, he/she can refer your daughter to a pediatric orthopedist, but it doesn't sound like that will be necessary.
i have a 15 month old boy that still walks on his toes intermentantly. my oldest son did the same and he out grew it...
my doctor seems to think they walk that way because they feel like it gives them better balance....don't worry...