Adobe publishes AMF3 spec
FDS, re-branded as BlazeDS is available under a LGPL v3 license. In their press release they also mention AMFPHP a couple of times, which is great news for the PHP community, because it implies they are backing the open source implementations.
I wanted to make sure the AMF implementation in SabreAMF is correct, by checking out the official spec. The spec is not really helpful though. It only covers AMF3, and missing some of the details. I guess I'm going to have to reverse engineer the 122MB behemoth that is BlazeDS.