What does BlazeDS Livecycle Data Services do, that something like PyAMF or RubyAMF not do?
Other than NIO (RTMP) channels, LCDS include also the "data management" features. 
Using this feature, you basically implement, in an ActionScript class, a CRUD-like interface defined by LCDS, and you get:

automatic progressive list loading (large lists/datagrids loads while scrolling)
automatic crud management (you get object locally in flash, modify it, send it back and DB will get updated automatically)
feature for conflict resolution (if multiple user try to updated the same record at the same time)
if I remember well, also some improved integration with the LiveCycle ES workflow engine

IMO, it can be very fast to develop this way, but only if you have only basic requirements and a simple architecture (forget SOA, that otherwise works so well with Flex). I'm fine with BlazeDS.
I'm doing a tech review and looking at AMF integration with various backends (Rails, Python, Grails etc).
Lots of options are out there, question is, what do the Adobe products do (BlazeDS etc) that something like RubyAMF / pyAMF don't?