FDS does not support RTMP tunneling. If using dedicated RTMP socket connections isn't an option you should use AMF polling. This is a feature area that we're doing investigation around for a future release.
Bear in mind that there are Flex 1.5 apps out there that use Flash Communication Server to do the data push bit of what in V2 is done by the RTMP support in FDS
Yes, you can use Flash Media Server to do data push using remote shared objects.
But data push to clients is only part of the FDS story. Remote shared objects hosted on FMS don't integrate as directly with J2EE domain models/Hibernate and JMS messaging as FDS does. If tight integration with a Java backend isn't a requirement, using Flash Media Server for push can be a great approach for your app.
Tell me, apart from streaming media into a Flex UI would there any reason to use FMS in a V2 application for data push or is it ALWAYS best in your opinion to migrate V1.5 apps (that use FCS/FMS for data push) to FDS2?
My advice is to "always" do what makes the most sense for your application ;) I'm a firm believer in the absense of absolutes.
That said, FMS should be used to stream AV to a Flex app because the FDS RTMP support is data-only. I wouldn't recommend porting an existing FMS application that Flex clients integrate with to FDS unless you had a good business reason to do so.
If you can envision the need to make RemoteObject calls against server-side Java resources, proxy web service or HTTP calls, or need to hook into JMS messaging or Java domain models using data management services I'd recommend FDS. Client to client data exchange can be accomplished with either server so your total set of requirements should drive your final decision.