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I'm constantly searching for this.
So far, the T-Level Infinity is the only thing decent I can find and you can't seem to get it in the UK anymore. I also agree that it falls down in other respects.
I also found this, but I'm not so keen on it:
http://www.boreasgear.com/products/bootlegger-eclipse-black
It's three different backpacks, with the frame/mesh harness swapping over to each of them. Only one of them is waterproof - the dry bag.
Is there such a thing as a waterproof bag that has the frame/mesh to lift it off your back? Most/all waterproof things I see look like they'd give you a hugely sweaty back.