Watching online TV from mac to Apple TV

Airplay mirroring will send (actually mirror) your Mac display to your TV screen and send (not mirror) audio to the TV. This means anything up on the Mac gets mirrored. You can use an ATV2 to mirror to but it will only output 720p, the ATV3 outputs 1080p. Yes you can JB the ATV2 and install XBMC on it and leave the MAC out of it, but as I said, you only get 720p.

I have both ATV2 and ATV3. ATV2 is Jailbroke and running XBMC. I never Airplay to it because my TV is 65 inches and a 720p desktop looks bad on it. I use the ATV3 to Airplay my desktop. I also use it to stream (not Airplay) 1080p movies I have in iTunes. I stream to the ATV3 because I have found that Airplaying video actually drops frames and compresses the video (read no buffering). Airplay streaming (not mirroring) does not compress the video or drop frames (read it buffers in a portion to start the vid).

With that said, and I could be wrong, you are probably better off with ATV2 and XBMC because there is not much 1080p content to be had on XBMC.

I honestly couldn't, nor could anybody else, tell you when and if the ATV3 will be jailbroken. It's taking a very long time, if anybody is even working on it, which does not bode well. But what do I know? Only this, the ATV2 is jailbroken today and if you can find one at a reasonable price then you struck gold. Don't buy an ATV3 thinking a jailbreak is inevitable.