Just a quick note for anyone out there who might be having problems connecting to their AirPlay/AirTunes speakers. Tonight I was attempting to play music over my AirPort Express, which has always worked in the past, with no luck. It kept erroring out with a -15000 error code. After doing some searching I found similar reports of issues but with iTunes 9 and different error codes but the resolution was the same: turn off IPv6.
The preference for that is in System Preferences->Network. Select your network device, then Advanced->TCP/IP. On my system IPv6 was set to configure automatically. I set it to off and my AirPlay/AirTunes magically started working again.
Not sure what the IPv6 setting was before but it is quite possible I had it set to off before and it got reconfigured when I installed a developer update to allow printing from the iPad.