Holy backache Batman! How much stuff do you plan to carry on your commute? I use an LL Bean ESCAPE daypack for my commute - 28 miles RT. I leave my shoes at work, and our work is business casual dress code. I can fit my clothes and a laptop in the bag. It's not specifically waterproof but I've ridden in downpours in it and it has kept my stuff dry. It is pretty low profile and doesn't create drag -- I've ridden with bigger packs and they are definitely a problem. It's not pricey either - $70 I think.

Use the smallest pack you can get away with...those things above look huge. I guess if yoru commute is short it might not make much difference.

Corrected -- thought my pack was the Ridge Runner...is actually an Escape. Not sure they still make it.