What does BlazeDS Livecycle Data Services do, that something like PyAMF or RubyAMF not do?
Good question. I'm not a ruby guy (i use java with flex), but what I believe differentiates blazeds vs commercial livecycle ds is

Streaming protocol support (rtmp) - competition for comet and such, delivering video
Some advanced stuff for hibernate detached objects and large resultset caching that I don't fully understand or need


support?
Might be others but those are the ones I know off the top of my head.
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?