I know someone who has one, and has been using it. But his immmediate target
is people with intellectual disabilities (he’s the guy behind
http://www.peepo.com – which incidentally should be accessible for blind
folks too, if not necessarily your choice of subject matter).

There is also a group in Columbia who have been working on accessible linux
applications for blind users on a PDA. They are the folks behind Blindux, a
linux distro with a simple set of applications, designed for ease of use over
extensive power. I’ll chase up the details (if I vanish, feel free to ping
me and remind me of this).

E-Mail from Charles McCathieNevile