We have just released the specification for AMF, as well as announced that we will be releasing and open sourcing a reference implementation (code-named BlazeDS).
From the labs page:
Previously available only as part of Adobe LiveCycle® Data Services ES, Adobe is announcing its plans to contribute the proven BlazeDS technologies to the community under the LGPL v3. BlazeDS gives the rapidly growing Adobe developer community free access to the powerful remoting and messaging technologies developed by Adobe.
You can find more information, as well as download the specification on labs.