Don't expect this as an iPhone app anytime soon. But...
Imagine this; a mobile phone that reads your lips.
The prototype of such technology called "electromyography" (in English that translates to "silent communication") was on display at CeBit in Germany last week. Cebit is Europe's biggest electronics tradeshow.
Electromyography is already in use by NASA (remember; they were the early adopters of Tang, too!).
Electromyography is able to read the most subtle facial movements and twitches to interpret what is being said, so that you don't even have to say it out loud.
The hope is that this would cut down on obnoxious people who overshare their phone calls in a loud voice in public. Having someone mouthe their conversations silently in public may be less obnoxious. But, I think it would be pretty disturbing and lead to a lot of unnecessary stroke scares.