Mumblez:

You saw that other pack of Seattle Sports at the rei outlet? It's 2 lb. 6 oz., which is under your 3 lb. limit, but a whopping 6,000 cu. inch (99 l.) capacity; probably way too big. It's a watersports bag that might be a good thing if you're hiking somewhere wet like the Juan de Fuca trail.

In that vein, this Arcteryx pack (18 oz., 2185 capacity - 40 l.), designed more as a secondary daypack to be pulled out on a long trek to say, summit an off-the-trail peak, is something along the lines of what you're looking for, but a bit small and 5 times the price of that fanny/backpack watersports thing of Seattle Sports. (BTW, I wonder how much of the fannypack's volume is taken up by the stored backpack when not in use?)

Seattle Sports also makes some neat dry bags for electronics for those hiking in perrenially wet areas.