SWEEET! I just installed software called accessx on the XO.
It allows a disabled user to adjust the way the keyboard behaves. For example, if someone can only hit one key at a time, they can enable the StickyKeys feature and then to get a capital letter, first they can press the SHIFT key by itself, then they can press the letter they want capitalized.
There are also ways to use accessx to control things like whether or not a letter will be repeatedly entered when its key is held down.
On an XO-accessibility mailing list I belong to, there is discussion about a text-to-speech synthesizer (so that everything a person types is pronounced audibly by the computer). I think the XO's version of this software is not quite ready for prime time, so I will hold off on installing it. But I'm all happy that I've taken one baby step toward setting up an accessible XO.