There's a new open source MagicPlay protocol that is suppose to emulate AirPlay functionality. Looks like it's available on the Raspberry Pi:
http://nanocrew.net/
It looks pretty cool. I don't know enough about Linux to make it work with Pogoplugs, but maybe someone on this forum would be interested in making it available. As with everything Arch, the many take advantage of the work done by the few. We appreciate it!