Yes, there is a group. Moodle Docs
"One of the main places accessibility
work is being carried out right now is on the Moodle Accessibility Collaboration Group mailing list, see http://collaborate.athenpro.org/group/moodle/
Why on earth we want to have the main discussion regarding Moodle accessibility outside Moodle, and not in it's own Forum on Moodle.org I'm not sure - seems important enough to me.
As to importance of usability - I would say it's critical (higher than high) and I think it's one of the 5 key areas for Moodle development now (if you can find any recent presentations by Martin accessibility is given great importance.
And of course many governments and education providers are having to take this into account now.
As an example, the new Atto editor in Moodle 2.7 actually has accessibility checking icons on the main toolbar
We have an accessibility statement on our main Moodle training site http://www.moodlebites.com/mod/page/view.php?id=13609
(it's not perfect, and it's still in development), but I think more and more Moodle sites will need to start thinking about this properly.