ah... its already been done. The price is right and it looks like the basic features are there.

http://reviews.cnet.com/8301-19512_7-10281062-233.html

Perhaps someone could leverage the iPhone platform to develop a hearing aid? Everything required is there... I think. =)

Its very useful. Here are a few of the ones I have come across.

Voice to text transcription of voicemails. You can read a message a lot faster than you can listen to one.

All email and voicemails can be emailed to you. Voicemails will have the sound file and the transcription

Reply to text messages through your email client

You can assign different missed-call messages for your different contact groups.

You can create a missed-call message for a single contact

You can record phone conversations

You can send specific callers directly to voice mail.

I have a Skype account and unfortunately Google Voice cannot use the Skype account to make this work on an iPod touch. Its a bit of a disappointment but there is a lot at stake in this space and progress comes slowly.