Aaron Leventhal of IBM has more details on the relationship between iAccessible2 and the UNIX/Linux accessibility API (ATK/AT-SPI) as well as the motivations behind iAccessible2’s evolutionary approach:

[A]n evolutionary path was needed for applications which already had MSAA (IAccessible) support… [and] an API was needed that did not require separate accessibility implementations for each platform.

[…]

The IAccessible2 interface itself collects important ATK features from other areas, as well some completely new methods and features… For the most part, features were added either to bring Windows capabilities up to the level of ATK/AT-SPI, or in order to support the features of ARIA (previously known of DHTML accessibility).

[…]

[W]hat we’re doing is expanding MSAA while matching ATK/AT-SPI to a very helpful degree. […] [B]ecause IAccessible2 is backwards-compatible with MSAA, the current support of Windows screen readers and other assistive technologies can continue to work on applications that add IAccessible2 support. However, the newer IAccessible2 capabilities will also be exposed, and thus newer assistive technologies will be able to take advantage of them.

Peter Korn of Sun weighs in as well.

(Via Andy Updegrove.)