I just posted this in a comment on an article about sending sign language video to handheld devices.
How about a sign language to text interface? It could use the familiar skills of the hearing impaired person to read the gestures (with fingertip markers like MIT’s Sixth Sense, or gloves with accelerometers) and output the text to a display on a T-shirt perhaps. This would allow easy communication with non-sign language proficient ‘normal’ people.
They could just use a keyboard I suppose, but this would use their existing skills and potentially offer a totally hands free interface.
Addendum: I worked at a service station once - 6 hearing impaired people rocked up wanting to buy some food and this one moderately good at speaking guy interpreted for the rest of them. It was hard (especially through the intercom/window). This idea could have been quite handy..