Arkel have recently introduced what must be the very lightest rear panniers: the 420 gr (for both) Dry-Lites. They are waterproof and feel surprisingly robust for their weight, and with their angled fronts they can be mounted relatively far forward for better handling without striking them with your heels.
They are claimed to be 32 litre bags (for two) but I fear that is a bit optimistic. However, with compact gear, the dirty and wet tent strapped on top, and perhaps a handlebar bag, they should suffice in many circumstances. I have tried them briefly, and was much impressed. I look forward to using them more seriously on a tour in the Swiss Alps this summer.
Willem