Long time gone from the blog, but here goes a resurgence.
Joe Valesko of Zpacks.com is introducing a new line of packs out of Dyneema X fabric come January 2010. Dyneema X is the current gold standard for those looking for UL gear made out of bombproof materials. If you pay him enough, Dan McHale will make you a pack made out of nothing but Dyneema strands.
Dyneema X weighs about 4 oz/yd^2. This is about three times the weight of silnylon (post-impregnation). However, when the amount of fabric in the gear is so minimal (like that on a pack), the weight increase is proportional but not overly substantial. I'd be willing to guess that a stripped-down Dyneema X 26 will weigh in at about 9 oz. This is three times the amount of my stripped-down Z1.
Other pack makers use, or have started to use, Dyneema regularly in their packs. GoLite, Six Moon Designs and MLD all use Dyneema is some fashion or another. However, none of them weigh in as low as the anticipated Zpacks models. The MLD 2010 Prophet is about the closest you're going to get for a full-featured pack (defined as a rucksack with straps, size pockets and rear pocket). GoLite's Ion is about 9 oz pre-strip down, but that pack lacks external pockets.
I'm slightly biased in preferences, of course, because I love the simplicity of my Z1. But if you want a UL pack for that needs to be tough, Joe's new line should be fantastic.