Gamasutra has a link up to an article on usability testing in games. This is a subject near and dear to my heart since major elements of games are basically "interfaces". If you assume that games are made up of token, verbs and rules, the first two speak directly to the 'interface' of the game and have a major impact on the title's playability.
Registration is required, but who isn't registered to Gamasutra? :-)
Click here to read the article