Ubuntu rolled out a new desktop notification service called notify-osd[1]. Currently the version in Jaunty alpha is not accessible since no ATK support was added. A version with ATK support could be found in my bzr branch[2].

The new notification bubbles also supports an integer value from 0 to 100 (-1 means that there is no value). This is used for indicators of scalar values, for example volume or screen brightness.

The a11y interface is implemented as a childless, top-level accessible with the role of ROLE_GLASS_PANE. The accessible's name is the notification bubble's title, the accessible's description is the bubble's text body. 

The bubble glass pane accessible also supports an Accessibility::Value interface. This reflects the bubble's current value. Displayed notification bubbles' values could be updated, so the object:property-change:accessible-value reflects this.

1. https://launchpad.net/notify-osd
2. https://code.launchpad.net/~eeejay/notify-osd/atk