nick: That would probably be referred to as a "buttpack" though there are other names as well.
I keep mine in BOTH my left rear and right rear pockets. I carry two different wallets -- one with cash and debit card (which is retrieved most often) and the other has ID, credit cards, business cards, etc. Both are thus rather small (ID one is less than 1/2 inch thick and it's the larger). When I read some articles saying that bulky wallets in one back pocket could lead to back problems, I started to worry. Plus, I just hate carrying a large wallet since it messes up those well fitted clothes.
I did use a money clip for awhile, but since I couldn't put my debit card in it (it's magnetic -- no more card!), I switched over to what amounts to a folded piece of leather with a metal bill holder/clip inside and a plastic covered pocket for my debit card on the outside. Works for me.