I was pointed to this article that talks about using AIM for conversations between hearing-imparied and non-hearing imparied individuals.  This allows the two parties to communicate without the use of  TTY terminal.

Still, there is a call center transcribing the information.  This seems like a really interesting application for speech recognition.  Assuming accuracy was high enough, this would remove the potential awkardness of speaking while there was a transcriber present.  I know there are other things to consider here, but to me, the link seems obvious.

http://www.betanews.com/article/AOL_MCI_Offer_Phone_Numbers_to_Deaf_with_IM/1102908638