All in a medium Osprey Atmos 35 backpack:
Lunch, Nalgene water reservoir, spare tube, tire patches, CO2 inflater with one cylinder, plastic tire irons, headlamp, a couple of blinking reflectors, sun and morning eyeglasses, eye drops/rinse, shirt and shorts, soft briefcase with PDA cell phone, camera, morning paper, work papers, mail from home, prescription meds, pens, etc.

The only items I have attached to my bike are a mirror, one water bottle cage, air pump, tail light, removable fenders, bike computer.

My commute to work is less than 13 miles one-way across a toll bridge and mainly downhill.