I have a Fedora 17 system with avahi configured to advertise a private service. The F17 is wired to a switch, to which is also connected a wireless AP. The client is wireless. The F17 firewall is disabled.
The switch is a simple domestic model from Netgear, as is the AP.
Under normal circumstances, clients do not see responses to MDNS queries sent using multicast to UDP port 5353, IP address 22.214.171.124, ethernet 01:00:5e:00:00:fb (or to IPv6 ff02::fb, ethernet 33:33:00:00:00:fb).
In trying to diagnose this I have used wireshark in the F11 host. Reproducibly, when wireshark is capturing, the clients see responses. When it is not capturing, they get no responses. The interface is being put into promiscuous mode while capturing but not otherwise. So it seems that promiscuous mode makes the interface see the packets. There must be some setting needed to let it see the packets the rest of the time.