As I always tell parents, our students cannot communicate their own original thoughts unless they either use a language compaction program like Unity/Minspeak (which, in my personal experience, can be limiting to literacy) or unless they can read and spell.
Reading and spelling has so many other benefits, as well as being a basic human right, that it is often the way to go. If you work with people of any age who use AAC to communicate you should check out the webcast about AAC and literacy at AAC-Rerc. It is never too late to learn to read.
PRC has been doing major work with literacy lately, now one of their competitors, Dynavox, has announced it will follow suit. The following new information was announced on Lon's podcast No Limit's to Learning "A.L.L (Accessible Language Learning) - A literacy program coming this fall (September 08) from Mayer-Johnson/Dynavox. Includes phonemic awareness, sound segmentation, etc. Provides guided and independent practice. Can be accessed on a Dynavox device, on Boardmaker plus or stand alone on a computer. The system compliments reading curriculums already in place as a supplement.