Just a note--the lightweight packs that most of us use for backpacking (wilderness trekking) are really not suitable for travel. If you've ever watched airline baggage handlers at work, you'll know what I mean!

For traveling I still use a convertible backpack from REI that I bought in 1992. It has back stays and a hipbelt so I can carry relatively heavy loads (even the 4 bottles of wine I bought in the south of France!). However, it has a pocket on the back into which I can tuck the hipbelt and shoulder straps, making it look like a normal suitcase. It also can be carried on most airlines. This is great for airline travel (nothing to catch on conveyor belts) and also on the occasions when I wanted to stay in a real hotel instead of a hostel. It still is in excellent condition, even though I travel a lot. I don't know if REI still sells these; the ones they're advertising with their current sale have wheels and weigh 8-9 lbs.