Why does the AirPlay icon disappear when I enable WPA2?

I have an early 2008 MacBook Pro set up to share its internet connection (ethernet) with my new AppleTV, new iPad, and Phone 4s by creating a WiFi network. When this network setup has no password, I can use AirPlay just fine on all my devices. But when I enable WPA2 protection for the network, I can connect to the network and browse the internet etc. on all my devices, but AirPlay is nowhere to be found! What's the solution?

All software on all devices is up to date. I've tried holding down option when creating the network to try WEP, no luck. WPA2, no luck (Is there a way to specify WPA2 Personal and not just WPA2 when setting up internet sharing?) No security, plenty of luck! But that's not what I want... Please help!