There is no firewall on the linux box, though I could install one. I have a Linksys WRT54GS router as the front end to the Internet, and pretty much depend on the NAT’ing to protect my network.
I have tried starting/stoping the mt-daapd server, and it makes no difference to the iTunes running on my WinXP laptop.
I downloaded mt-daapd_0.2.4-1_i386.deb from Sourceforge and installed that on my Kubuntu 6.06 (Dapper Drake) installation. I’m not sure what you mean as the mdns backend, can you be more specific? In looking up mDNS on Google it looks to me like the Zeroconf thing provided by Kubuntu does the mDNS work, though configuring it with Kubuntu seems like it doesn’t do very much.