I have a handicapped friend, and we have been trying to get a good smartphone solution working for him. He's currently got an HTC EVO, and as many people have said, Android's voice capabilities are pretty amazing (although Android has a separate called "Voice Dialer" which is absolutely terrible for some reason). One problem he's had, however is touchscreen confirmation. Siri looks like it will fix a lot of problems, and allow him much more freedom.
Also, I'd like to give a shoutout to the folks developing Tecla ( http://scyp.idrc.ocad.ca/projects/tekla ) for Android. It's an open source wheelchair integration for Android that uses the Arduino to interface with whatever controls the user currently has. I found it in my search for a solution, but my friend wanted to hold off until he gets a new chair. Regardless, Tekla is easily the coolest mobile open source project I've discovered, and I just wanted to give those guys credit. The project page is https://launchpad.net/meadl
While iOS 4 provides great accessibility features, it mostly excels in providing support for visually impaired users. As a touch interface, it has certain mobility requirements that Jimmy didn't like. He would have had to mount the phone inches from his face (where he keeps his non driving hand). Siri seems to greatly decrease those mobility requirements, and he is excited.
Also, I'd like to give a shoutout to the folks developing Tecla ( http://scyp.idrc.ocad.ca/projects/tekla ) for Android. It's an open source wheelchair integration for Android that uses the Arduino to interface with whatever controls the user currently has. I found it in my search for a solution, but my friend wanted to hold off until he gets a new chair. Regardless, Tekla is easily the coolest mobile open source project I've discovered, and I just wanted to give those guys credit. The project page is https://launchpad.net/meadl