There are two questions that came up repeatedly at the conference:
A key to accessibility of interactive web controls is their support for the W3C's draft Accessible Rich Internet Applications (ARIA) specification. ARIA addresses the accessibility of dynamic web controls by defining a method by which developers can specify roles and states of custom HTML widgets. Supporting browsers pass this information on to supporting assistive technologies.
There doesn't seem to be much talk about accessibility within the MooTools or Prototype/Scriptaculous communities, which I suspect would indicate inaccessible code, but I'd love to be corrected. Another framework, Ext JS, has published a Ext JS Roadmap which includes "ARIA/Section 508 accessibility improvements" among the features slated for inclusion in the Ext JS 3.0 release, scheduled for Winter 2008 or Early 2009.
The following is a summary of frameworks that from my perspective seem to be taking accessibility most seriously:
Dojo was the first framework to have substantial accessibility support. The core widget set of Dojo has included accessibility support since the release of Dojo 1.0 in 2007, including full keyboard support, ARIA support, and support for high contrast mode. According to the Dojo A11Y Statement, they are still "the only fully accessible open source toolkit for Web 2.0 development." That may be true, but I think one or two of the others may be catching up.
Dojo also includes extensive accessibility documentation in its on-line manual, The Book of Dojo. Becky Gibson of IBM's Emerging Technologies group is Dojo's accessibility lead, and she presented an excellent and unbiased paper at the 2007 W4A conference in Banff titled Enabling an Accessible Web 2.0 (in PDF). I spent a few days hiking solo among grizzlies and elk prior to that conference, but that's another story...
The jQuery Accessibility community is actively working to build accessibility support into jQuery. One significant development was Chris Hoffman's jARIA plug-in, which extended the jQuery object with a set of ARIA methods. In an announcement posted to the community on September 22, David Botter says that ARIA functions have now been integrated into jQuery UI's core. This would seem to be a very promising new development.
Yahoo! User Interface
Yahoo! has been adding accessibility to its widgets since 2007 if not earlier, as documented in their blog post Using WAI-ARIA Roles and States with the YUI Menu Control. However, their support for accessibility still seems to be isolated, although slowly expanding. In a blog post on October 2 titled ARIA Plug-Ins for YUI Widgets, they announced that "a handful of widgets" now include ARIA support.
Google Web Toolkit (GWT)
For my own development efforts, I've getting my feet wet with jQuery, and as time permits I may have a look at Dojo and YUI as well. If others have firsthand knowledge of accessibility of a particular framework, please share your stories and observations!