The Acrobat 9 SDK is posted to the Acrobat Developer Center for public download.
But wait, there's more.
New in Acrobat 9, we've added a livedocs version of the documentation. Now you'll be able to provide feedback on most topics and even submit corrections or report bugs. Now's the chance to show off your Kung-Fu skills and contribute to the developer community. Comments will be monitored so don't expect them to show up right away but they will show up.
Flex Developer Alert!
Acrobat 9 Custom Navigator SDK is available in Pre-release:
Use Flex and your ActionScript skills to create SWF interfaces (Navigators) for PDF Portfolios using the new ActionScript APIs for Acrobat 9. Check out my previous post to see some of the possibilities. The good news is, the Custom Navigator SDK is all ActionScript, you don't need to know anything about the rest of the Acrobat SDK to get started.
Use this PDF form to request access to the prerelease program.