If you've always fancied an iPhone but wasn't sure if you could manage it, you maybe interested in the new Accessibility features of iOS5.(iPhone's operating system)
As well as helping people with motor difficulties by finding ways around the multi touch gestures, you can (if you still have good speech) control your phone entirely by speech which may also be beneficial for those with sight difficulties.
There is a video on YouTube though it was filmed with the BETA version (pre-release version).
It's demoed on an iPad on here but this video has best explanation (I've been told), the iPhone version does the same.
Also.. did you know for people who already have one and struggle with the inbuilt qwerty keyboard for texting.. if you go into Settings>General>Accessibility and turn Zoom on when you tap with 3 fingers at once you can make the qwerty keys bigger and easier to hit, you just slide it around to get to letter you want.
If hitting the screen with 3 keys at once is a problem download iOS5 and go into Accessibility and turn on Assistive Touch. you then get a little dot on screen this brings up menu's you can hit with one finger/thumb/mouthstick the can select 2 fingers gestures, 3 finger gestures and use that to activate Zoom with just one digit/stylus etc.
If you have an iPad you can also get iOS5 by connecting it to your registered computer and updating the operating system (of the iPad, not the computer!) and use it the same way!