Today I was given a link to a really great existing application for autistic people or others with a speaking disability like some people with the Down syndrome. It is called Proloquo2Go which is latin for speak out loud. Proloquo2Go is an application for the iPhone, iPod Touch and iPad. In this movie you can see what it really does:
Here you can also find a nice interview about it by ABC news on YouTube (embedding was disabled) which explains the use some more.
Website application: http://www.proloquo2go.com/