I see this is an old post, but:
We recently bought the Pogey travel cot for our then 6 month old baby daughter. It is extremely light and easy, and folds nicely to fit inside a suitcase (a backpack might be more problematic).
It comes with a mattress, but on cold floors (fi tiles) or in a tent, I would put something thicker under (thermarest fi).
It also features a no nonsense mosquito net.
I don't know how long we will be able to use the Pogey for, as at 7 months, our daughter is already pulling herself up from her crib. I would definitely never use this unattended.
so:
+: cheap (around 20£), light, small
-: not sure how long you can use this, depends on your baby
http://www.babyworld.co.uk/information/reviews/product.asp?id=470