jQuery does very few things ... but does it well, in a simple manner and cross-browser. There are no complex widgets, plugins or whatever that could be considered as RIA in the library itself. It provides the bricks to build them.
Complex widgets that can be used out of the box by webdevelopers and webdesigners are found in libraries like jQuery UI or jQuery Tools. These libraries with complex scripts like tab menus, slideshows, etc can be whether utterly inaccessible or manage ARIA role, states and properties without having to worry about it.
Scripts from http://hanshillen.github.com/jqtest/ are based on jQuery UI plugins that have been improved for keyboard and ARIA support and any accessibility problem that could exist in the original scripts.