In reply to leeoftroy: I dont think there is any way to waterproof it, although i suspect a lot of the fabric will already be waterproof, but not taped zips and big zips everywhere, personally i go for the using dry bag options someone mentioned,
I think ortilab (sp?) make some proper rusksacks, that are waterproof, as apposed to a dry bag with straps if that makes anysense? How ever id probably still use dry bags just to help organise a bit!
With your current bag, you could try the silicone spray, breathabillity is less an issue as with clothing, you could also shove a load of wax over the zips but it will still let in somewhere! Nikwax is a water based product, it maintains breathabillity but wont have a good a barrier as a silicone product!