If you are watching Netflix on a non-PC device (i.e., Wii, Google TV, etc.) then use your PC to search for titles and place them in your instant que, works much better.
I haven't looked for any Backpacking related documentaries, but there are several great ones set in the Himalayas and Antarctica.
I did watch a really long one recently about two guys who hiked hiked along most of the Amazon River (floated the rest); took something like 2 years to complete, it was a great watch. Can't remember the name.
Post any good finds you catch here, please.