Valve's Gabe Newell appears to have recently conducted a focus group with deaf individuals in order to learn more about sign language and how it can be applied to games, notably the upcoming Half-Life 2: Episode 3. Why? To help develop Alyx's character, and further the animation technology implemented in the Half-Life games. Newell explains: "[Alyx] had a crush on someone who was hearing impaired, so she taught Dog how to sign so she could practice. Something happened, maybe, the person is off fighting the Combine someplace else, but that's why she and Dog would start signing with each other when they wanted to communicate without making noise, or communicate without other people knowing ... It gave us the excuse to build the technology for signing."
The focus group responds by noting the importance of facial expression whilst signing. While the Half-Life franchise has been known for its believable facial animation, it might take even more to fully realize the nuances necessary for authentic signing. It'll be interesting to see if Valve is up to the challenge.