I think that fact that all of the widgets that come with GWT are
created by JavaScript running in the browser, I think it unlikely that
any will comply. GWT also makes heavy use of tables for "panel"
widgets, another no-no for accessibility.
About the best that I can offer are the widget-wrapper (wwraper
package) in the GWT Widget Library (http://gwt-widget.sourceforge.net).
It lets you write your HTML code and then basically hijack the HTML
elements and make them widgets.
With some work this allows you to do things like make an app that works
with or without JavaScript enabled. You would write your app without
JavaScript, then write GWT code that enhances the capabilities. When I
say "enhance" I mean replace page refreshes with RPC, or replace
pop-ups with tooltips or floating divs.
This has come up before on the list, but I don't know if any progress
has been made as far making it easier to make GWT accessible.
Rob