What about things like Boxee or XMBC? Or VLC say
My assumption w/ Mountain Lion--and we're just guessing at this point--is anything that you can open up and play on your Mac desktop you can Airplay to the Apple TV...because that functionality is going to be built right into the OS itself, Mountain Lion will automagically
detect the presence of an aTV2 on your network and allow the little Airplay icon to show up...presumably just like how an iPad handles "display mirroring." On the iPad, even when you're using an app like Netflix which specifically does not
support AirPlay from within the app, you can still start playing something with the Netflix app on the iPad, then invoke display mirroring via double-tap of the home button for the iOS menu bar Airplay icon and bingo, it's fullscreen on your Apple TV. (Not that you'd ever need to do this since Netflix is supported on the Apple TV, but the principle is what I'm after.)
So I expect VLC would be just like playing Flash or Hulu or Netflix within Safari or the Apple dvd player or any other standalone desktop player, you start watching something, then select AirPlay mirroring to the aTV2. As long as you're using a Mac with a Sandy Bridge or better processor, that is.
XBMC (Boxee is dead on the Mac) is a little different in the sense that it is a fullscreen front end, if you're already "within" its user interface there wouldn't be a way to invoke AirPlay mirroring unless that was specifically added by those devs (i.e. as long as Apple first gave them a way to add it) or there was a way to watch XBMC in a window rather than fullscreen. (There may be, I just don't know the command since I've only ever wanted XBMC to be fullscreen.) If you could run XBMC in a window on the desktop, then sure, you'd be able to Airplay that to an aTV2.
How reliable is video streaming via AirPlay?
From the iPad, very reliable to a wired Apple TV2, which is the only way I've used mine. I do it a lot, it's the main way I watch non iTunes/iOS ready content on my stock Apple TV2 (via AirVideo on the iPad.)
...to play some media that you don't want to transcode for iTunes/iOS compatible content?
yes, most of us here have a pretty good Mac right at our HDTVs, so we can very easily play anything we want already with XBMC right up to the 1080p of our TVs, but for 1) a secondary location where you only have the budget for a $99 device this new feature will be nice for someone 2) who doesn't already have an iPad (because if you have one you've already been doing live transcodes with AirVideo and AirPlaying video to the aTV2 at 720p for the past year and you haven't needed a Sandy Bridge Mac to do it.)
Of course, it's limited to 720p because Apple TV is limited to that resolution?
Even though I just picked up another refurb aTV2 from Apple the other day, I'm more eager than ever to see if an Apple TV 3 is released in addition to the iPad 3.