Multicast in Rackspace cloud
Multicast isn't currently allowed on Rackspace Cloud Servers. The traffic volume of multicast traffic ends up overloading the switching layer and it degrades network performance for all traffic.
I heard it was a way to have multicast with openVPN... using TAP instead of TUN.
So it could works on both ec2 and rackspace, and you ensure that just your clients are allowed to subscribe to multicast group because only your clients are allowed to use your openVPN.