It's a great idea to develop something for the visually impaired but I do see one flaw. How is someone that cannot see going to find the app in the menu, much less know to tap the top corner or the bottom middle if they cannot see? Does the application have a voice feature to tell the users where to tap? If not it would be a great idea to add it.
You ask how does a person blind find the app? With no audio feedback, a person can remember the location by some tactile pattern on the area, by it, etc.
I am sighted and have worked with AT for the Blind since 1989 and so I have thousands of customers. Everyone of them I know loves the iPhone and although it has it's imperfections most customers appreciate the access.
When you build accessibility into an operating system you have an advantage much like the Apple OS versus Windows. Although Windows 7 has improved magnifier, narrator and the voice recognition, the world of windows is playing catch up to apple in regards to built in accessibility. On the other side, Windows has awesome add-on products to accommodate a person with a disability.
I appreciate Nokia working on making accessibility become built in, as opposed to an add-on like Talks and Mobile speak. I appreciate the add on, but the future is in having everything built in, OCR, ebook reader, TTS, voice rec. While accessibility products are special today, someday we all will talk to the box and the box will talk to use no matter what hardware is crunching the binary language.