I don't see why the MBA would not be able to play back HDM at 1080p on a 50"+ screens. IT supports larger resolutions than 1080p as it supports the 23" CD.
What are you planning on watching? Clips or full 1080p movies? If you consider that a BD can hold up to 50GB of data, the MBA HD will probably not even hold the contents of even 1 full disc with software and OSX also on the drive. Especially so for the SSD version...
If you are talking about HD movies off the iTunes then absolutely. Thats what it screams out to be used for.
As a sidenote, I really wish that apple would stop pushing highly compressed downloadable HD movies. Once you watch HD movies in full 1080p on a 1080p screen with DTS HD MA, there really is no gong back!
Regards,
C