With the release of W40K: Apocalypse and with Games Day coming soon then GW have a limited edition Apocalypse bag that's meant to look like an army backpack with the Cadian VIII symbols on it, or for the Chaos worshippers then there's the Volscani IV.
TBH I'm not greatly impressed with it. It appears to fit a single large carry case (which any true Guard player would be ashamed to manage to fit his army in) and the beige/tan colour just looks terrible. Isn't Cadia supposed to predominantly green? I've seen some beige fatigues, but always with green armour and green backpacks.
On the plus side, at least you're only being charged £11 for the rucksack itself, with all of the other components being £49 of the £60 price tag.