my friends and i were just pondering this since we like bike touring and hiking / camping. we came up with something similar to carbonfiber - the REI flash packs 50 and 65 are plenty good for a 2 - 3 day hiking trek. it can be stripped down by leaving the internal structure and top compartment at home getting the pack down to about 2 pounds.

we were thinking of each bringing one rolled up in a stuff sack and strapped to the top of our racks and biking to a trailhead and then hiking / camping for a couple days. we'll let you know how it goes as far as real world experience.

as you say: it would also be good for long trips using different types of transportation. sometimes you want to put all your gear / panniers in one easy to carry bag at train stations / hostels etc...