Someone will correct me if I'm wrong: As I understand it, MC can't talk to AirPlay devices without third party software like AirFoil. I'm not sure what you're seeing, but I don't think that AirPlay devices are accessible directly as Core Audio. If they were then third party players that are not made by Apple partners could talk to them. For example, if you could use VLC, or Vox, or another non-apple-partner software player to send to AirPlay devices, then MC would work too. But I'm pretty sure you can't.
If you want more attention to this question, you might ask this same again in the Media Network forum:
http://yabb.jriver.com/interact/index.php?board=7.0MC *can* talk to most DLNA devices; that support is built in. The main "gotcha" with DLNA is audio sync. If you want to play to several devices with the *same* audio at the same time, MC can't really guarantee they will stay in sync. AirPlay, on the other hand, has sync built in to the protocol, so they always stay synced. For some people, this is no big deal. For others, it is a must have feature.
Good luck.
Brian.