Until now. Luckily the hardware on my MacBook Pro supports Intel's Quick Sync technology, so with the recent release of OS X 10.8 it is able to do AirPlay Mirroring to an AppleTV. Which would be great... if I had an AppleTV (and I've almost considered buying one just for this). But given that I've already got a MacBook plugged into my A/V set-up it would be more convenient to use that.
I tried a couple of free AirPlay server applications on the MacBook (both AirPlayer and BananaTV allowed media to be sent from Caroline's iPhone to the TV (BananaTV worked better with audio)) but they didn't allow AirPlay Mirroring from my laptop. But when I downloaded the paid AirServer app I found that it did exactly what I wanted. I'm less than a day into the week long free trial, but unless I find some serious flaws I'll be purchasing it.
Update 2012-09-24: There was a security update for my MacBook, which required a restart, which ended my free trial. But I purchased AirServer, and am happy with it. There are some problems with video/audio synchronisation when sharing the screen from the MacBook Pro to the MacBook (I found that setting the Audio Buffer Size to 2200 seemed to give the best synchronisation), but the best solution for streaming video is to do it from iTunes, and then the video and audio stay in sync.
[*] My previous best solutions were just to use a cable (3.5mm for just audio, HDMI for video/audio), although for audio only setting up AU Lab 2.1 to send the System Audio (intercepted by SoundFlower) to another instance of AU Lab running on the target system worked with the lowest latency. Setting up a reverse VNC connection worked well enough to allow displaying of photos to the computer plugged into the TV, but both of these are a bit fiddly compared to just plugging a cable in, or the wireless simplicity of AirPlay.
