Some more observations in case it helps someone else.
The manifestation of the problem for me is that the ATV disappears from the AirPlay list as viewed from both iTunes on a Windows 7 PC and an iPad 3. Other AirPlay devices (two Airport Express [wired]) remain visible at all times.
The ATV disappears from AirPlay when it goes to sleep but it also disappears whilst it is active (light on) and I'm playing music through it. It carries on playing however even though it's not on the list.
If the AirPlay selection in iTunes is put in "Single" mode it does not disappear. Moreover if left in single mode it is able to wake the ATV up even after it has been asleep for a day.
Before 5.2 the ATV was always visible, even when asleep, in the AirPlay list on both iTunes and iPad and either could wake the ATV up.
For me the solution is to set sleep to never and put it to sleep manually when I'm not using it. Then when wanting to use it again do a restart to properly wake it up. Annoying but workable.
My network could not be simpler - wired PC and wired ATV - both with fixed IP - and only a switch between the two.
I appreciate there might be something somewhere I could change somewhere on my network to make it work ... although I can't imagine what and I do this for a living ... but that would just be masking what is clearly a bug with 5.2 ... quite simply because it used to work and with no change other than the 5.2 upgrade now it doesn't.