Every time I get a new wallet, I spend much more time than I should on attempting to find the perfect wallet for my needs. I must admit I have a small butt, and thick wallets make me uncomfortable when I'm in bureaucrat mode. It turns out that the wallet alone is 60% of the bulk, since I rarely carry more than my cards and papers with me. I rarely carry bills, let alone change, and I don't need to keep any order for 5 plastic cards.
I then stumbled upon a post about Dynomighty's Mighty Wallet. Unlike other wallets, this one is made from the same thin plastic used in Tyvek envelopes, making it slim enough to go unnoticed. On top of being incredibly thin, they come in a variety of cool designs. This YouTube video covers everything you need to know about it.
Since the shipping is ridiculously expensive, you might consider ordering them off eBay, where they sell for about 18$, shipping included.
UPDATE: I've received my wallet and have used it for a few weeks. It's barely noticeable in my backpocket and has a very durable construction. While it does not have separate pockets for every card, it makes up for it by being astoundingly compact. Did I mention it looks damn good, too? I have the Elysium model, but there are plenty of wicked designs available. Go and get one!