Room for two on that fence, Si? Yes, it all depends on the circumstances. At various times I've taken most of the things other people have mentioned, though not all at once, and trekking poles, socks and umbrella too. As a bare minimum, I'd take 300ml of water, a muesli bar, waterproof jacket, bandanna, binliner, knife and drugs (nurofen, imodium, antihistamine, iodine). I rarely bother with a thick fleece or waterproof trousers.
When we did a 5 day hutting route in Austria last summer, the airport weighed my pack in at just under 6kg, though I was wearing most of my clothes at that point.
A recent thread covered this from a slightly different angle: http://www.outdoorsmagic.com/forum/forummessages.asp?URN=2&UTN=806&SP=588450434115466703436&V=1