I use the Brunton Solaris 26. This is a fold out 26 Watt panel that weighs 28 oz. It's not light, but in my limited testing it reliably charges smaller electronics (iPhone, AA NiMH Battery Charger). It folds out to 22"x38" so hanging it off a backpack will likely be too cumbersome.
I've found this rule to work: "Find out your max power needs, then buy something that promise at least twice that output." A better rule seems to be to set a budget and try to get as much power output you can afford.
I also charge a Lenovo S12 (a 3lb/12 inch screen netbook) with a 3 cell 11.1V 2.4AH battery off the panel. It's questionable if this is wise since the open circuit voltage on the panel is 19-23V, and the notebook wants 20V (input from those with more electronics experience is *really* appreciated :). On a overcast day, I got about 20 % charge off 2 hours of charging.
Solaris 26 product page:
A great review, by a soldier that used it in Iraq: