Two backpacks...

Lowepro Orion Trekker II (medium) http://products.lowepro.com/product/...II,1989,14.htm
Tamrac Expedition 7 (big) http://www.tamrac.com/5587.htm

The Tamrac has all my gear in it and I only take it on big trips (10+kgs fully loaded).
The Lowepro I pack with what I need for a shoot if not taking the whole kit.

For the Tassie trip next year I'll bring both.
As we will have our car - the Tamrac keeps everything in one place safe and the Orion is good for specific bits during the day.