Below is a video of a discussion between Gabe Newell, a game developer at Valve, and a focus group including deaf and hearing impaired gamers. Valve is conducting research to gather information needed in order to incorporate signing into Episode 3 of Half-Life 2.
In the video, the importance of the use of realistic facial expressions was discussed, as well as other issues, such as deaf culture.
(All three video clips related to Gabe's focus groups can be found on the Escapist website.)
(I'm a little behind the game accessibility news...)