There are some silnylon packs/drybags with padded straps etc that are available. They generally are very lightweight but not very durable and are somewhat overpriced. There are quiet a few packs available for the ultralighters from companies from;

Gossamer gear.

http://gossamergear.com/packs/backpacks.html

Golite

http://www.golite.com/Product/ProdDetail.aspx?p=PA5264&mc=176&t=&lat=

Granite Gear

http://www.granitegearstore.com/Vapor-Trail-P8C7.aspx

Outdoor Research

http://www.outdoorresearch.com/site/dry_peak_bagger.html

Alpkit

http://www.alpkit.com/shop/cart.php?target=product&product_id=16363&category_id=251

Overall the packs require much more thought when packing and usually require using a sleeping pad to act as back padding to give the pack some shape and stability.