If you try to access Microsoft's DirectX 9.0a Download Page, you'll simply receive the following error message:
The download you requested is unavailable. If you continue to see this message when trying to access this download, you might try the "Search for a Download" area on the Download Center home page.
Also, any references to the recent release in Microsoft's Download Center has been removed.
It is not clear at the moment why Microsoft has decided to erase all evidence of 9.0a's existence.