Airtunes speakers and iTunes are best friends and they don’t want to let anyone else play with them.
Well tough because now Airfoil is playing the good parent and making Airtunes play nice with everyone else too.
Install Airfoil on Mac OS X or Windows XP / Vista and you can select any application to grab the sound from and send it to your Airtunes speakers.
You can even install Airfoil Speakers and make your computer pretend to be a set of Airtunes speakers.
All very clever and certianly worth the $25 single user (but not single computer) license.
My favourite uses are currently Airtunes + last.fm and also asking the whole office for a cup of tea via my macs microphone.