This is something that I've been thinking about for a long time.
I feel that pure weight loss will have a positive, negative, or neutral effect on your backpacking, depending on how you do it. Watching TV and drinking UltraSlimFast? Unless you're overweight or obese this will probably weaken you as much as it lightens you. Starting regular exercise regime and eating less Trans Fat? You will feel like superman on the trail; even more than if you buy a $500 tent that weighs 1 lb less.
Unless you are overweight or obese, I don't think that "weight loss" is what you need to be focusing on: I believe that FITNESS is absolutely the key to ultralight backpacking. A gym membership should be considered the most basic piece of UL Backpacking gear.
Increased fitness yields:
Greater ability to keep oneself warm, day and night
Less sweating = less wetting out of clothes and pack
Ability to move fast in response to approaching weather
More restful sleep
Less need for sleep
Less need for breaks that cool you off and break your rhythm
Less need to adjust clothing
Ability to carry less water
Ability to carry less food
Less stress on the body = improved appetite and digestion
Less fatigue = better decisions, quicker reactions
Better balance + co-ordination = fewer accidents and mishaps
Better mood, better attitude
More hours on the trail per day!
Yes: if all the time I spent on BPL were devoted to working out, I would probably be capable of more miles than I am today. I love UL backpacking as mental exercise: it's a logic problem, and an exercise in proactivity and memory and self-reliance. It's also better for joints like the knees that were *not* designed for the stresses we put on them. However, now that I am below 10 lbs base weight I focus my time and energy increasingly on my full-body strength, endurance, flexibility, and especially cardiovascular fitness.
EDIT: PS consider the extreme fitness sought by soldiers of all types — a SEAL team needs to spend 2 days and 2 nights swimming and walking 100 miles with a 100lb load on no sleep – just to get to the fight. Then they have to complete their mission and get out. Imagine what those guys could do with a ULA Conduit and a Gatewood Cape! :D