Just looking over this issue since @quicksketch rightly closed it - #193887: Accessibility enhancements
HTML5 now includes longdesc and it looks like it will have more support for all users in FireFox in the future. The debate on that (and long list of recommendations about why it's needed), is here:
https://bugzilla.mozilla.org/show_bug.cgi?id=854848
There's less of a discussion of this in Chrome, but it's here:
http://code.google.com/p/chromium/issues/detail?id=224285