According to BetaNews, “most notably corrected in version 9.0a is an issue that caused MSN Messenger to malfunction when DirectX 9 was installed.
It is not known at this time why DirectX 9.0a was removed from Microsoft’s download site, which currently states “The download you requested is unavailable.”
DirectX 9.0 made its debut last December featuring improved performance and security, along with a new high-level shader language designed to simplify development. However, graphics chipsets and games have only recently begun to take advantage of the DirectX 9 APIs.
Although not listed on the DirectX Web site, version 9.0a was formerly available for download via Web installer. Microsoft notes the release also corrects problems with some multiplayer games and resolves other minor bugs. “