My granddaughter is 7 months old. She locomotes by flipping on her back, arching her back, pushing with her feet, and scooting on the top of her head. She can go quite fast in this mode and clearly prefers it to crawling on her belly, which she can do for short distances. Has anyone else seen this? Should I be concerned?

Normal. Children learn in different ways. She found it easy, so she does it

Some children roll to get where they want, some children scoot. This is your daughter's way of crawling, for right now. She may get it right, she may not, but as long as she is reaching her developmental milestones, there should be no reason for concern.

Hello All - Just FYI - I spoke with my daughter yesterday. She took her baby to the doctor and was told NOT to allow the baby to crawl in that way (belly up with back arched, pushing with feet and scooting on the top of her head) anymore because she could hurt herself.

That's good, the way you described it sounded like she could hurt her neck.