I have been looking for a solution for some time and a recent chance visit to the Rohan shop in Covent Garden, London saw me finding something that will definitely work well for me.
The Dry Safe Organiser by Rohan is a travel organiser made from waterproof materials. I suspect it is meant for those going on holiday or outdoor types but it fits my needs perfectly.
The dimensions are 18.5cm x 10cm x 2.5cm so it is quite compact. It is made from a 'Hyperbaric polyamide' which is waterproof and the welded seam aids it waterproofing prowess. It is also a strong material that is tear resistant.
Inside there is a large compartment with space for up to eight credit cards, a large zipped pocket and two organiser sleeves. Apart from all of this, I bought it as I am able to safely stow my iPhone inside, thus keeping it safe and waterproof. I have a small neoprene pocket that my phone fits into and will probably use that too to protect against knocks while this organiser sits in my bag / rear jersey pocket.
As far as weight goes it must only weigh a few grams and certainly wouldn't upset the sensitivities of even the strictest weight-weenie!
While looking at this I also saw that Rohan sold a small waterproof pocket that would fit some case and credit cards. This would be great for times when I am not on the bike.
I am pretty pleased with this set up. The camera case solution or wrapping my phone in a freezer bag, wasn't one and this makes me a great deal happier, especially with events like the Dunwich Dynamo where packing light/efficiently is a good idea.