I am the Public Relations person for Benchmade Knife Company. As a backpacker myself I stumbled upon this forum and wanted to throw in my two cents.
When I first started working at Benchmade my mind immediately started thinking of which knives would be the best for backpacking. I have actually gone down to multiple REI's and trained their staff at the knife counter on different types of steels, handle materials, strength thresholds and more.
From the Benchmade line there are a few suggestions that I have that fit the bill for both lightweight and high quality:
The 530 Pardue - http://www.benchmade.com/products/product_detail.aspx?model=530
This lil fella is 1.88oz, has high quality and durable 154CM steel (for those of you not familiar with steel this is consider high quality because of the edge that it will maintain, ease of sharpening, semi-stainless steel and strength)
The 10610 Benchmite - http://www.benchmade.com/products/product_detail.aspx?model=10610
The main advantage of this folding knife is that it is even lighter and smaller at 1.60oz. Good quality AUS8 blade steel and a fantastic price point.
The 556 Griptilian - http://www.benchmade.com/products/product_detail.aspx?model=556
This knife was recently featured in the 2007 Backpacker Magazine's Gear Guide. It received some of the highest accolades from the editors and there is a good reason. Combine patented technologies, form, function, durability, reliability and price... then you have this knife. It is not as light as the rest but trust me, adding the extra .80oz will be WELL worth it.
I hope this information helps. Feel free to throw any questions back my way. I would love to answer any questions anyone might have regarding steel, components, handle materials, weight, etc...