As announced by Thibault Imbert updated SWF and AMF specs are available for download at http://www.adobe.com/devnet/swf.html. I’ve included the what’s new text at that page for the new versions below. A link to previous version 10 SWF specs is also kept at the page as archive.
SWF File Format Specification (version 19)
The SWF 19 specification includes bug fixes as well as the EnableTelemetry metadata tag. The SWF file format is available as an open specification to create products and technology that implement the specification. SWF 9 introduced the ActionScript 3 language and virtual machine. The SWF 10 specification expands text capabilities with support for bidirectional text and complex scripts with the new DefineFont4 tag. TheDefineBitsJPEG4 tag allows embedding JPEG images that have an alpha channel for opacity and also a smoothing filter. SWF 10 also adds support for the free and open-source Speex voice codec and for higher frequencies in the existing Nellymoser codec. A new EnableTelemetry tag was introduced in Flash Player 11.4 and that affects all SWF versions.
Action Message Format (AMF) Specification
Action Message Format (AMF) is a compact binary format that is used to serialize ActionScript object graphs. Once serialized an AMF encoded object graph may be used to persist and retrieve the public state of an application across sessions or allow two endpoints to communicate through the exchange of strongly typed data.
With specs updated and no feature classified as premium (currently), kudos to Adobe, it seems Flash is taking steps to become an open format again.