I've done a bit of work on my iPad Mini with iOS 8.0.2 - using the BBC iPlayer app (which streams video from the BBC in the UK, and will allow Airplay from within the app via an icon).
My wifi network is created by 2 Airport Expresses (both publishing their own audio Airplay targets), and also contains a couple of Apple TV 3rd generation models. Crucially, one of them is a so-called Revision A model which is the one with the shrunken A5 chip as opposed to the original model which has one deactivated core. Both had been updated to the version 7 AppleTV software.
An iPhone 4 running iOS 7.1.2 works as before - you can Airplay anything on screen to any device in the network (including audio to either of the airport expresses).
An iPod Touch 4 running iOS 6.1.6 will do the same.
All devices, including the Mini, are on the same wifi network, none of them have bluetooth switched on as a rule.
The iPad Mini used to Airplay happily until it was upgraded to iOS 8.0 and then to iOS 8.0.2 - the Airplay icon disappeared from most apps capable of using Airplay.
Crucially, iPlayer has been updated by the BBC in the last few days but there's no difference - it's as though the iPlayer app is unaware of any Airplay Devices.
In iOS 8.0.2 you can swipe up to get a control panel that lets you see AirDrop and (eventually) Airplay. This didn't seem to work consistently.
After much messing about I turned bluetooth on in the iPad, and that brought back the Airplay icon but crucially it would only see one device - the Rev A Apple TV.
This led me to believe that turning on the bluetooth was letting the iPad see the AppleTV as though it wasn't connected to the home network (but it was, as I could stream video though iPlayer and browse web pages).
This doesn't seem to work in other apps which would have presented an Airplay icon in iOS 7.1.2 but don't in iOS 8.0.2 and crucially haven't been updated recently to be compatible with iOS 8.
It's as though the iPad Mini assumed that it couldn't see any suitable Airplay devices despite being on the same network, but turning bluetooth on allowed it to use Peer to Peer discovery and finally see the AppleTV.
The fact is, though, that the iPad really was on the network but didn't seem to realise it for Airplay purposes and only people who own the Revision A Mark 3 Apple TV could spot that.
Things have been rebooted, and wifis forgotten in methodical ways to try and isolate the issue based on other forum posts with no discernible difference.