I've been using a Eusebio backpack first for for 8 months commuting on the pushie then last year on the motorbike.
*edit*
It's called a: Eusebio Sport - Cross Country 30
It's been awesome, I don't even notice it's there when riding.
It's a special backpack for active trekking/ walking/running. Hard pipe frame around the outside, extremely well padded, 2 straps around the front, one at stomach one at chest. These take most of the weight off the shoulders and the stomach ones have handy pockets in them.
It's bright orange, highly water resistant with reflectors, and a self containing water proof cover in bright yellow.
It's got straps all over it to pull it in to almost nothing on your back, and can expand to very large volume. There's a netting on the back to store more things in, I take shoes in there, and I've even carried a large midi keyboard in it with one occy strap!

i expand the straps to put it over a guitar on my back when need be.
Will buy again in a heartbeat when I need to.