I agree that a backpack worn over a suit jacket or sport coat is a jarring sight. I see it all the time in my industry (commercial aviation) but it still doesn't come across as the most professional approach to me. That said, I've come around to the fact that backpacks are just more convenient for moving through airports, especially when pulling a golf travel case behind.

I've just recently moved to an eBags Professional Slim backpack. The backpack straps can be tucked away completely out of sight and there is a horizontal carry handle. When put in this configuration, the only indication it's not any other laptop bag is the slight taper in thickness from one end to the other. I wish there was a tuck-away single carry strap, but the instances where I'd need to carry it horizontally are usually only from the car into the customer's office and back out, so I can live without it. It also has a rolling back handle pass through, so I can keep it off my shoulders/jacket (wrinkles) most of the time but have fully padded shoulder straps when they're needed (i.e. pulling a golf bag case through the airport).