Ryan -
While I was at Drexel, we ran one vlan for their 802.1x ssid. There were 4 subnets on the VLAN. It consisted of a bunch of /20, /21, and /22. I don't remember the specifics. As much as Aruba hated that we ran the network this way, there weren't any problems as long as we kept broadcast/multicast disabled.
Mobility seemed to be fine, as you could roam any where there was an access point and maintain the same L2 connection. With regards to subneting, you were never guaranteed to be in the same subnet. We didn't run multicast, so I can't speak to that.
With regards to the mDNS stuff, we didn't support that either. But, if two devices are in different L3 networks but on the same L2 network, then I don't see any problems. The recieving device (laptop) would be able to resolve the service device (AppleTV), and then be able to send traffic to it via the router.