There are apps that do this, though I don't know how well.
This one is currently available, but my hunch is it just uses standard voice to text, so it'll probably be shoddy:
http://www.livecaptionapp.com/
And this one is new, currently in beta, and looks more promising since it handles multi-person conversations pretty slick-like and has a team that seems pretty cool behind it:
http://www.transcense.com/
Coupled with glass, these could be even cooler. But even as-is on a phone, assuming they actually speech to text well, it could be great.