In my setup I'm using mt-daap/Firefly as the DAAP server, its openly available for Linux requiring Avahi as the mDNS component to work on a network with Apple's Bonjour.

While I like iTunes on OSX/XP as client I wouldn't use it as a DAAP server.

Rokulabs, for one, are supporting Firefly development as their Soundbridge players support DAAP and their forum lists a load of compatible clients.

So there's def opensource support for client & server apps.