I bought some of the white widow spectra from Seattle Fabrics several years ago, too. The selection of fabrics there is decent, and they often carry a few hard-to-find items, but they are just a clearinghouse. They know little to nothing about what they sell. I copied this description of the White Widow Spectra from the website, when they had it in stock:
"This is a 500 denier nylon with a Spectra carbon filter ripstop reinforcement woven into the fabric. The white carbon fiber ripstop grid gives this fabric a unique look and an ample coating makes it nice and waterproof."
They also assured me that their heat-sealable 70d nylon can be sealed on both sides (it clearly can't). Also, if you order anything from them online, no matter how small (even just a buckle or some buttons), you will be charged $16-$40 for shipping. You have to call in your order and ask them to manually calculate the shipping cost for it. So, be aware of these issues if you use Seattle Fabrics.
Nathan, the packs look great. They look simple, tough, and well-designed. How much does the framesheet on the bigger pack weigh?