I assume you get no joy with other apps that use AirPlay like YouTube? Is DHCP turned on on the router? This allows dynamic IP addresses to be assigned. I would turn everything off and reboot in order waiting each time for things to fully come back to working state before turning on the next device.. Modem>Router>Denon>iPhone
Quick update if anyone is following this... Feel a little embarrassed as it turns out not much was wrong other than a lot of conflicting devices... I chose to turn everything off completely and try Turing on in a clear sequence to work out the source of the problem. Anyway, the long and short of it is that my laptop which was on with iTunes. It seems to be that was conflicting with all the devices and I had thought at one point iTunes was needed for AirPlay to work. Apparently not... Anyhow... I can't tell you how pleased this has made me feel. I am now totally in love with the iPad app... Thanks ....
Finally got this.... Not intuitive really .... Even after reading this thread i struggled to work out what. I didnt see the airplay icon in the mini popup when you toych the song playing album art... As someone else said it needs the full page artwork pucture up...
I'm confused still as well. I have an Iphone 4 and an Ipad and have no problem contecting to my apple tv via either. I do not get an airplay icon on either device even when douple tapping the album art. One of the main reasons I signed up for a premium account was to be able to stream to my apple tv and my stereo system. Getting frustrated as I've spent over an hour trying to figure it out.
In my instance it was an issue with having my laptop with itunes running on and I believe this was conflicting with my amplifier.
I've also experienced a problem if I switch iOS devices playing Spotify using AirPlay. For example. Switching from an iPad to an iPhone. However, this is usually resolved by switching off the amplifier so that it refreshes itself.
If in doubt, switch everything off and turn on just one AirPlay device at a time. Worked for me and I buggared around for ages.