Get your copy of the Nielsen Norman Group Report Beyond ALT Text: Making the Web Easy to Use for Users With Disabilities - for free. (At least it is free as I write this - normal price 124 USD.)
I have yet to read the report myself, but have been advised by reliable sources that it is an excellent piece of work.
Get them while they're 'ot, they're luvverly!
Published Saturday, 22nd December 2007.
Smiffy at LinkedIn