I bought a seriously light pair of Adida runninf shoes, like 7oz for size 12. They aren't the trail runners, but I think I will use them on the trail after I am done road and track running with them.

The trail runners are heavier at 10oz for size 9:
http://www.rei.com/product/772537
which isn't that remarkable for trail runners as there are many under 12oz in size 12.

Then I got to thinking about making some homemade moccassins, and building in some foam or just leaving room for thicker foam insoles, and then maybe peeling the tread off of trail runners when the rest of the shoe falls apart and shoe-gluing that on to the bottom of the mocs. I have a pair of light leather hiking shoes now that are only 16oz in size 12, and with a lower ankle and lighter construction I'll bet maybe 10oz even 8oz per shoe is doable, depending on what sort of rocks you want to be able to jump on and how much weight your carrying. I like the feel of leather. Its a good material.