I carry a simple, leather Samsonite bag that fits even more than what you listed and fits under seats quite well. It has a strap on the back to loop over the handle of your roll-aboard and also has a shoulder strap. I've used it for both travel and to carry things to court for trial.

It generally carries the following when traveling:

14" Acer Aspire Timeline in a 15.4 sized BYO sleeve
Large Bottle of GNC Ultra Mega Gold multi-vitamins
Canon 30D Digital SLR
Various magazines
A notebook
Charging cable for laptop
Extra batteries and a charger for the camera
Large zipper travel wallet with passport and BPs.
Extra USB cables
Extra earbuds
A protein bar or two.
3 extra pairs of underwear (usually serving a dual purpose of being wrapped around the camera for extra protection).
Baekgaard Toiletry bag (front zip for easy Freedom Baggie access)

It also looks better than a backpack and I paid about $50 including free shipping on eBags for it.