The poor-man backpacker's solution to this is lining your backpack with a heavy-duty garbage bag and then twisting the top shut before closing up your bag. This will keep your clothes and sleeping bag dry unless you throw your pack in a river. I use this technique along with a standard rain cover for redundancy, and hey, you never know when you might need that trash bag for something else. Things that absolutely can't get wet (electronics, etc.) go in a separate fully watertight case. I think I'm going to move to Aloksaks for this purpose.

As an aside, I _did_ throw my day pack with no waterproofing in a river once. It was retrieved in less than 10 seconds, and surprisingly, the contents were bone-dry. My friends still won't let me live that one down.